Add file tree sidebar and title-bar toggle

[Dimillian]

Motivation

• Provide a read-only file tree view for projects so users can inspect files without opening an IDE.
• Let users switch the right-hand Git panel between the existing Git view and the new file-tree view from the title bar.

Description

• Added a new FileTreePanel component (src/features/files/components/FileTreePanel.tsx) that builds an expandable folder/file tree from list_workspace_files results and shows a context menu action to Reveal in Finder.
• Wired a new filePanelMode state through the app: added filePanelMode and onToggleFilePanel in src/App.tsx, a title-bar toggle button in src/features/app/components/MainHeader.tsx, and selection logic in src/features/layout/hooks/useLayoutNodes.tsx to render FileTreePanel instead of GitDiffPanel when appropriate.
• Re-used the existing useWorkspaceFiles data (files list) and @tauri-apps/api/@tauri-apps/plugin-opener menu APIs to present a macOS Finder reveal action.
• Added styling for the file tree in src/styles/file-tree.css and imported it in src/App.tsx.

Testing

• Started the dev server with npm run dev and verified Vite served the app successfully.
• Loaded the app in a headless browser via Playwright and captured a screenshot of the UI that includes the new file-tree panel, which completed successfully.
• No unit or typecheck commands were executed as part of this change; manual UI smoke tests were used for verification.

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Codex Task