Replace Mosaic split-view library with custom tiles

[Mng-dev-ai]

Summary

• Replace Mosaic library with custom grid-based tile system for managing split panes
• Simplify layout state management using 2D array (rows of tiles) instead of binary tree
• Remove 1000+ lines of abstraction while maintaining feature parity
• Remove Mosaic from package.json dependencies

Changes

• Rename mosaicHelpers → tileHelpers with new layout utilities
• Refactor uiStore split-view state to use TileId[][] grid layout
• Replace MosaicSplitView component with WorkspaceSplit
• Update all layout hooks (useActiveViews, useIsSplitMode, etc.) for tile layout
• Update type definitions and command registry

Test plan

• Verify split panes can be opened side-by-side (row layout)
• Verify shift-click stacks panes vertically (new rows)
• Verify pane closing removes tiles and preserves remaining layout
• Verify mobile view shows single pane switching
• Verify secondary (split-chat) tiles work correctly
• Check that existing split-view state restores on reload