feat: Redesign review tab with phase selection, commit navigation, and consolidated toolbar

- Add BranchManager.listCommits() and diffCommit() for commit-level navigation
- Add getPhaseReviewCommits and getCommitDiff tRPC procedures
- New ReviewHeader: consolidated toolbar with phase selector pills, branch info,
  stats, integrated preview controls, and approve/reject actions
- New CommitNav: horizontal commit strip with "All changes" + individual commits,
  each showing hash, message, and change stats
- Slim down ReviewSidebar: file list only with dimming for out-of-scope files
  when viewing a single commit
- ReviewTab orchestrates all pieces in a single bordered card layout

docs/frontend.md

@@ -111,10 +111,12 @@ The initiative detail page has three tabs managed via local state (not URL param
 ### Review Components (`src/components/review/`)
 | Component | Purpose |
 |-----------|---------|
-| `ReviewTab` | Review tab container with diff viewer and preview integration |
-| `ReviewSidebar` | Review info, actions, file list, comment summary |
+| `ReviewTab` | Review tab container — orchestrates header, commit nav, diff, sidebar, and preview |
+| `ReviewHeader` | Consolidated toolbar: phase selector pills, branch info, stats, preview controls, approve/reject actions |
+| `CommitNav` | Horizontal commit navigation strip — "All changes" + individual commit pills with stats |
+| `ReviewSidebar` | File list with comment counts, dimmed files when viewing single commit |
 | `DiffViewer` | Unified diff renderer with inline comments |
-| `PreviewPanel` | Docker preview status: building/running/failed with start/stop |
+| `PreviewPanel` | Docker preview status: building/running/failed with start/stop (legacy, now integrated into ReviewHeader) |
 | `ProposalCard` | Individual proposal display |
 
 ### UI Primitives (`src/components/ui/`)