feat: major TUI improvements with clipboard, cell mode, and configuration system

This commit introduces significant enhancements to the TUI application:

Clipboard functionality:
- Vim-like yank operations (y, yy, yc, ya) for copying data
- Cross-platform clipboard support using arboard crate
- Copy cells, rows, columns, or all filtered data
- Status line shows last yanked content

Cell mode vs Row mode:
- Toggle between cell and row selection with 'v' key
- Cell mode for copying individual values (e.g., trade IDs)
- Row mode for traditional record-based operations
- Visual highlighting adapts to current mode
- Yellow foreground highlighting for better readability on dark themes

Configuration system:
- TOML-based configuration file support
- Configurable glyph vs ASCII mode for better compatibility
- Theme customization including cell selection styling
- Behavior settings (auto-execute, case sensitivity, etc.)
- Keybinding configuration support

UI improvements:
- Cleaner, mode-specific status line
- Shows pinned column count in status
- Removed clutter from navigation mode
- Better visual feedback for current operation

Buffer system design:
- Architecture for multi-file support (not yet integrated)
- Independent state preservation per buffer
- Tab-based interface preparation

Documentation:
- Added CELL_MODE.md explaining selection modes
- Added CONFIG.md with configuration details
- Added BUFFER_DESIGN.md outlining buffer architecture

These changes optimize the tool for professional technical workflows,
particularly for copying identifiers between systems and working with
multiple data sources.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: TimelordUK

sql-cli/BUFFER_DESIGN.md

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+# Buffer System Design for SQL CLI
+
+## Overview
+
+The buffer system allows multiple files to be loaded simultaneously, with each maintaining its own independent state. This is similar to vim's buffer system or browser tabs.
+
+## Key Features
+
+### 1. Independent State Per Buffer
+Each buffer maintains:
+- Loaded data (CSV/JSON)
+- Current SQL query
+- All filters and searches
+- Cursor position and scroll state
+- Sort order
+- Column pins
+- View preferences (compact mode, row numbers, etc.)
+
+### 2. Quick Switching
+Switch between buffers without losing any state:
+- Your position in the data
+- Active filters
+- Current query
+- Everything is preserved
+
+## Keybindings
+
+### Buffer Navigation (Vim-style)
+- `gt` or `Ctrl+Tab` - Next buffer
+- `gT` or `Ctrl+Shift+Tab` - Previous buffer  
+- `{n}gt` - Go to buffer n (e.g., `2gt` for second buffer)
+- `Alt+1` through `Alt+9` - Quick jump to buffer 1-9
+
+### Buffer Management
+- `:e filename.csv` - Open file in new buffer
+- `:edit filename.json` - Same as :e
+- `:bd` or `:bdelete` - Close current buffer
+- `:ls` or `:buffers` - List all open buffers
+- `:b {n}` or `:buffer {n}` - Switch to buffer n
+- `:bn` or `:bnext` - Next buffer
+- `:bp` or `:bprev` - Previous buffer
+
+## Visual Design
+
+### Tab Bar
+```
+┌─────────────────────────────────────────────────────┐
+│ [1:trades.csv] [2:products.json*] [3:customers.csv] │
+├─────────────────────────────────────────────────────┤
+│ SELECT * FROM products WHERE price > 100            │
+└─────────────────────────────────────────────────────┘
+```
+
+- Current buffer is highlighted
+- `*` indicates modified/filtered state
+- Number prefix for quick jumping
+
+### Status Line Addition
+```
+[NAV] Row 45/1000 | 📁 FILE | Buffer 2/3 | Enter:Run
+```
+
+## Implementation Strategy
+
+### Phase 1: Basic Buffer Support
+1. Extract current state into Buffer struct
+2. Add BufferManager to handle multiple buffers
+3. Implement basic switching (gt/gT)
+
+### Phase 2: File Operations
+1. Add :e command to open files
+2. Implement :bd to close buffers
+3. Add duplicate file detection
+
+### Phase 3: Visual Polish
+1. Add tab bar widget
+2. Show buffer indicators in status line
+3. Add buffer list view (:ls)
+
+## Use Cases
+
+### Comparing Data
+```
+1. Load trades.csv
+2. Filter to specific date range
+3. :e reference_data.csv
+4. Search for specific values
+5. gt to switch back to trades
+6. Your filter is still active!
+```
+
+### Multi-File Analysis
+```
+1. Open main dataset
+2. :e lookup_table.csv for reference
+3. :e config.json for settings
+4. Use Alt+1, Alt+2, Alt+3 to quickly jump between them
+```
+
+## Benefits
+
+1. **No Context Loss** - Each file keeps its state
+2. **Quick Comparison** - Rapidly switch between datasets
+3. **Familiar Interface** - Vim users will feel at home
+4. **Efficient Workflow** - No need to restart for new files
+
+## Technical Considerations
+
+### Memory Management
+- Each buffer holds its own data copy
+- Consider max buffer limit for large files
+- Lazy loading possible for unused buffers
+
+### State Preservation
+- All UI state moves into Buffer struct
+- App becomes primarily a buffer manager
+- Clean separation of concerns
+
+### Ratatui Compatibility
+- Fully compatible with ratatui's architecture
+- Tab bar is just another widget
+- State management remains straightforward
+
+## Future Enhancements
+
+### Advanced Features (Not Initial Scope)
+- Split panes (view 2 buffers side by side)
+- Buffer groups/sessions
+- Save/restore buffer sessions
+- Quick buffer switching with fuzzy finder
+
+### Integration Ideas
+- Open query results in new buffer
+- Compare buffers with diff view
+- Transfer filters between buffers
+- Linked scrolling for comparison
+
+## Command Examples
+
+```sql
+-- In buffer 1 (trades.csv)
+SELECT * FROM trades WHERE date > '2024-01-01'
+
+-- User presses :e products.csv
+-- Now in buffer 2 (products.csv)
+SELECT * FROM products
+
+-- User presses gt
+-- Back in buffer 1, filter still active
+
+-- User types :ls
+-- Output:
+1: trades.csv (filtered)
+2: products.csv
+3: customers.csv
+
+-- User types :b3
+-- Switches to customers.csv
+```
+
+This design keeps the tool focused on its core purpose (simple SQL queries over flat files) while adding powerful multi-file capabilities that don't complicate the interface.

sql-cli/CELL_MODE.md

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+# Cell Mode vs Row Mode
+
+SQL CLI now supports two selection modes, similar to csvlens, optimized for different workflows.
+
+## Quick Start
+
+Press `v` to toggle between Cell and Row mode.
+
+## Cell Mode
+
+**Purpose**: Focus on individual cell values, perfect for copying IDs or specific values to other systems.
+
+### Visual Indicators
+- Only the selected cell is highlighted
+- Status line shows `[CELL]` mode indicator
+- Current cell value displayed in status line
+- Cell highlighted with yellow text, bold and underlined (by default)
+
+### Key Behaviors
+- `y` - Instantly yanks the current cell value
+- Navigation moves cell by cell
+- Clear visual focus on exactly what will be copied
+
+### Status Line Example
+```
+[NAV] [CELL] Row 45/1000 | Col: platformOrderId = TRD-2024-0042 | Yanked: dealId=DEAL-789
+```
+
+## Row Mode (Default)
+
+**Purpose**: Traditional row-based selection for working with entire records.
+
+### Visual Indicators
+- Entire row is highlighted with dark gray background
+- Status line shows `[ROW]` mode indicator
+- Row selection arrow `►` shows current row
+
+### Key Behaviors
+- `y` - Enters yank mode (yy for row, yc for column, ya for all)
+- Navigation moves row by row
+- Column highlighting shows current field
+
+## Customizing Cell Highlighting
+
+The cell highlighting style can be customized in your config file to work better with your terminal color scheme.
+
+### Default Style (Yellow Foreground)
+```toml
+[theme.cell_selection_style]
+foreground = "yellow"
+use_background = false
+bold = true
+underline = true
+```
+
+### Alternative Styles
+
+#### Orange/Amber for Gruvbox
+```toml
+[theme.cell_selection_style]
+foreground = "yellow"  # Often renders as orange in Gruvbox
+use_background = false
+bold = true
+underline = false  # Less visual noise
+```
+
+#### High Contrast
+```toml
+[theme.cell_selection_style]
+foreground = "black"
+use_background = true
+background = "yellow"
+bold = true
+underline = false
+```
+
+#### Subtle Style
+```toml
+[theme.cell_selection_style]
+foreground = "cyan"
+use_background = false
+bold = false
+underline = true
+```
+
+#### Minimal
+```toml
+[theme.cell_selection_style]
+foreground = "white"
+use_background = false
+bold = true
+underline = false
+```
+
+## Color Options
+
+Available colors for `foreground` and `background`:
+- `"black"`
+- `"red"`
+- `"green"`
+- `"yellow"` (often orange in many themes)
+- `"blue"`
+- `"magenta"`
+- `"cyan"`
+- `"white"`
+- `"gray"` / `"grey"`
+- `"dark_gray"` / `"dark_grey"`
+
+## Use Cases
+
+### Cell Mode - Copying IDs
+1. Load trades data
+2. Press `v` to enter cell mode
+3. Navigate to the `tradeId` column
+4. Press `y` to copy just that ID
+5. Paste into another application
+
+### Row Mode - Analyzing Records
+1. Load data (default row mode)
+2. Navigate through records
+3. Press `yy` to copy entire row
+4. Or press `yc` to copy entire column
+
+## Tips
+
+1. **Quick ID Copy**: In cell mode, `y` immediately copies - no need for `yy`
+2. **Visual Clarity**: The highlighting makes it crystal clear what will be yanked
+3. **Status Feedback**: Always shows what was last yanked
+4. **Terminal Compatibility**: If colors are hard to read, customize the style in your config
+
+## Troubleshooting
+
+### Cell Highlighting Hard to Read?
+
+If the default yellow text is hard to read on your color scheme:
+
+1. Generate a config file if you don't have one:
+   ```bash
+   sql-cli --generate-config
+   ```
+
+2. Edit `~/.config/sql-cli/config.toml` (Linux/Mac) or `%APPDATA%\sql-cli\config.toml` (Windows)
+
+3. Try different color combinations:
+   - For dark themes: `foreground = "cyan"` or `foreground = "magenta"`
+   - For light themes: `foreground = "blue"` or `foreground = "red"`
+   - Disable underline if it's too busy: `underline = false`
+
+4. Restart SQL CLI to apply changes