feat: add /issues/recent tab — flat list sorted by most recent updates

Shows all issues in a single flat list ordered by updatedAt descending,
without grouping by status. Displays the update timestamp instead of
creation date in the trailing column.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/App.tsx

@@ -42,6 +42,7 @@ export function App() {
         <Route path="issues/active" element={<Issues />} />
         <Route path="issues/backlog" element={<Issues />} />
         <Route path="issues/done" element={<Issues />} />
+        <Route path="issues/recent" element={<Issues />} />
         <Route path="issues/:issueId" element={<IssueDetail />} />
         <Route path="goals" element={<Goals />} />
         <Route path="goals/:goalId" element={<GoalDetail />} />