feat(FN-982): add branch selection and commit viewing to Git Manager

- Add branch commit fetching API endpoint (GET /api/git/branches/:name/commits)
- Enhance BranchesPanel with selectable branches and commit diff viewer
- Add CSS styles for branch selection list and commit detail view
- Fix relativeDate utility to handle invalid/empty date strings gracefully
- Fix interview route ordering in mission-routes
- Add loading state for branch commit diff to prevent false error flash
- Add comprehensive tests for branch selection, commit viewing, and route handling

packages/dashboard/app/api.ts

@@ -869,6 +869,12 @@ export function fetchGitBranches(): Promise<GitBranch[]> {
   return api<GitBranch[]>("/git/branches");
 }
 
+/** Fetch recent commits for a specific branch */
+export function fetchBranchCommits(branchName: string, limit?: number): Promise<GitCommit[]> {
+  const query = limit ? `?limit=${limit}` : "";
+  return api<GitCommit[]>(`/git/branches/${encodeURIComponent(branchName)}/commits${query}`);
+}
+
 /** Fetch all worktrees */
 export function fetchGitWorktrees(): Promise<GitWorktree[]> {
   return api<GitWorktree[]>("/git/worktrees");