feat: surface git context in sidebar and chat header

- Add GitContextBadge to sidebar ProjectGroup showing branch name,
  dirty indicator, and ahead/behind counts for real projects
- Show git-missing and error states with subtle warning badges
- Add hover-reveal refresh button per project to re-scan git status
- Show branch/dirty/ahead/behind inline in ChatPane header subtitle
- Wire refreshProjectGitContext callback from App through Sidebar

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/renderer/App.tsx

@@ -265,6 +265,9 @@ export default function App() {
             onSetSessionArchived={(sessionId, isArchived) => {
               void api.setSessionArchived({ sessionId, isArchived });
             }}
+            onRefreshGitContext={(projectId) => {
+              void api.refreshProjectGitContext(projectId);
+            }}
             workspace={workspace}
           />
         }