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

apps/server/git/branch-manager.ts

@@ -6,7 +6,7 @@
  * a worktree to be checked out.
  */
 
-import type { MergeResult } from './types.js';
+import type { MergeResult, BranchCommit } from './types.js';
 
 export interface BranchManager {
   /**
@@ -45,4 +45,15 @@ export interface BranchManager {
    * since local branches may not include all remote branches.
    */
   remoteBranchExists(repoPath: string, branch: string): Promise<boolean>;
+
+  /**
+   * List commits that headBranch has but baseBranch doesn't.
+   * Used for commit-level navigation in code review.
+   */
+  listCommits(repoPath: string, baseBranch: string, headBranch: string): Promise<BranchCommit[]>;
+
+  /**
+   * Get the raw unified diff for a single commit.
+   */
+  diffCommit(repoPath: string, commitHash: string): Promise<string>;
 }