feat: search text across files

[lezli01]

Summary

• add a dedicated, resizable sidebar view for workspace-wide text search
• search Markdown, JSON, and YAML files recursively through a non-blocking Tauri command
• group matches by file with relative paths, line numbers, and text previews
• open a selected result in MarkPad and move the CodeMirror cursor to its line
• support Ctrl+Shift+F on Windows/Linux and Cmd+Shift+F on macOS
• document the feature with current light and dark screenshots

Why

MarkPad users with folders of notes and data files currently have to open documents one at a time to find references, headings, tasks, links, or values. This adds a local-only folder search workflow while keeping the initial scope focused and dependency-free.

Closes #114.

Validation

• npm test — 270 tests passed
• npm run lint
• npm run build
• cargo fmt --all --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo check --all-targets --all-features
• cargo test — 2 workspace-search tests passed
• README audit — all local images present
• browser visual check — search toolbar action, sidebar empty state, dark/light themes, and Cmd+Shift+F verified