feat(file): add file opening workflows — native dialog, Cmd+O/Cmd+P

[khang859]

Summary

Mission 4 of the File Editor feature: adds three UI entry points for opening files.

• Cmd+O: Native file dialog (multi-select, no type filter, starts in current CWD) — opens selected files as editor/image tabs
• Cmd+P: Quick open overlay with fuzzy file search — shows recent files initially, searches sector directory on type, arrow/enter/escape navigation with highlighted matches
• Command palette: "Open File..." and "Quick Open" commands listed with their shortcuts
• Recent files: tracked in workspace store, persisted to localStorage (max 20), shown in quick open when no query entered; stale entries are naturally handled by the OS (non-existent files won't open)

Implementation

New IPC channels

• file:open-dialog — calls dialog.showOpenDialog() with multi-select, starts in provided defaultPath
• file:list — lists files recursively; uses git ls-files (respects .gitignore) with fallback manual walker

Files changed

• src/shared/constants.ts — two new IPC channels
• src/main/ipc-handlers.ts — handlers for both channels
• src/preload/index.ts — expose file.openDialog() and file.list()
• src/renderer/src/lib/shortcuts.ts — open-file (Cmd+O) and quick-open (Cmd+P)
• src/renderer/src/lib/commands.ts — two new commands
• src/renderer/src/store/workspace-store.ts — recentFiles state + addRecentFile(), localStorage persistence
• src/renderer/src/hooks/use-pane-navigation.ts — keyboard handlers for Cmd+O/Cmd+P
• src/renderer/src/components/QuickOpenOverlay.tsx — new component
• src/renderer/src/App.tsx — wire event listeners and render overlay

Test plan

• npm run build passes ✅
• Cmd+O opens native file dialog; selected files appear as new tabs
• Cmd+P opens quick open overlay showing recent files
• Typing in quick open fuzzy-matches files from current directory
• Arrow keys navigate, Enter opens file, Escape closes
• Opened files appear in recent list on next Cmd+P open
• "Open File..." and "Quick Open" appear in command palette (Cmd+Shift+P)

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