Add staged/unstaged sections and file actions to Git diff panel

[Dimillian]

Motivation

• Improve the Git diff sidebar to clearly separate staged and unstaged changes and allow quick file-level actions (stage/unstage/revert) from the UI.
• Surface index vs worktree status so users can operate on files without leaving the app.

Description

• Backend: split Git status into files, stagedFiles, and unstagedFiles, added helper run_git_command, and implemented Tauri commands stage_git_file, unstage_git_file, and revert_git_file in src-tauri/src/git.rs and exported them in src-tauri/src/lib.rs.
• Service layer: added wrappers stageGitFile, unstageGitFile, and revertGitFile in src/services/tauri.ts and extended getGitStatus return type to include stagedFiles/unstagedFiles.
• UI: updated the Git panel to render separate "Staged" and "Unstaged" sections with counts and per-file context menus that expose "Stage file", "Unstage file", and "Revert changes" actions in src/features/git/components/GitDiffPanel.tsx; added handlers wired through src/App.tsx and the layout hook useLayoutNodes.
• Styling: added section header and list styles in src/styles/diff.css for the new sections.

Testing

• Ran npm run lint (ESLint completed; warning about unsupported TypeScript version from @typescript-eslint was shown but no errors). — succeeded
• Ran npm run typecheck (tsc --noEmit) — succeeded
• Started the dev server with npm run dev and captured a UI screenshot of the Git diff panel to verify staged/unstaged sections and menus — succeeded

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Codex Task