Implements the hunk-level actions promised by the 1.13.0 changelog entry. Each expanded file diff with 2+ hunks gets a Hunks menu (next to the file actions) listing every hunk with its +/- counts and Stage/Unstage + Discard buttons, wired to the existing stageGitHunk/unstageGitHunk/ revertGitHunk API and POST /api/git/apply-hunk backend.