refactor: extract window runtime

[PathGao]

Fixes #236.

Summary

• Move process-wide window state, file watchers, window-state persistence, secondary-window creation, single-instance delivery, and window event handling into window_runtime.rs.
• Keep Tauri command names, payloads, builder wiring, and frontend behavior unchanged.
• Leave lib.rs responsible for top-level builder and command registration.

Merge order

This is the first PR in the replacement chain:

1. window runtime extraction (this PR)
2. window session extraction
3. document session extraction
4. replacement for feat: move tabs between windows, merge all windows, and a carry shortcut #225
5. replacement for feat: window tags — named color chips, with pinned tags as reopenable sessions #226

Validation

• npm run check
• npm test (117 passing)
• cargo test (18 passing)

[PathGao]

@alecdotdev This starts the replacement chain for #225 and #226. It is behavior-preserving: runtime ownership moves out of lib.rs while commands, events, and payloads remain unchanged.