Improve Active Workers table and recover stale tasks

Backend:
- Add _recover_stale_tasks() to reset tasks stuck in 'in_progress'
  from previous crashes (tasks >5 min old get reset to pending)
- Called automatically on startup

Frontend:
- Fix dark mode colors using CSS variables instead of hardcoded values
- Add elapsed time column showing how long task has been running
- Add spinning indicator next to package name
- Add status badge (Running/Stale?)
- Highlight stale tasks (>5 min) in amber
- Auto-updates every 5 seconds with existing refresh

frontend/src/pages/AdminJobsPage.tsx

@@ -212,25 +212,43 @@ function AdminJobsPage() {
                           <th>Version</th>
                           <th>Depth</th>
                           <th>Attempt</th>
-                          <th>Started</th>
+                          <th>Elapsed</th>
+                          <th>Status</th>
                         </tr>
                       </thead>
                       <tbody>
-                        {activeTasks.map((task) => (
-                          <tr key={task.id} className="active-row">
-                            <td className="package-name">{task.package}</td>
-                            <td className="version-constraint">
-                              {task.version_constraint || '*'}
-                            </td>
-                            <td>{task.depth}</td>
-                            <td>{task.attempts + 1}</td>
-                            <td className="timestamp">
-                              {task.started_at
-                                ? new Date(task.started_at).toLocaleTimeString()
-                                : '-'}
-                            </td>
-                          </tr>
-                        ))}
+                        {activeTasks.map((task) => {
+                          const elapsed = task.started_at
+                            ? Math.floor((Date.now() - new Date(task.started_at).getTime()) / 1000)
+                            : 0;
+                          const minutes = Math.floor(elapsed / 60);
+                          const seconds = elapsed % 60;
+                          const elapsedStr = minutes > 0
+                            ? `${minutes}m ${seconds}s`
+                            : `${seconds}s`;
+                          const isStale = elapsed > 300; // 5 minutes
+                          return (
+                            <tr key={task.id} className={`active-row ${isStale ? 'stale-row' : ''}`}>
+                              <td className="package-name">
+                                <span className="spinner"></span>
+                                {task.package}
+                              </td>
+                              <td className="version-constraint">
+                                {task.version_constraint || '*'}
+                              </td>
+                              <td>{task.depth}</td>
+                              <td>{task.attempts + 1}</td>
+                              <td className={`elapsed-time ${isStale ? 'stale' : ''}`}>
+                                {elapsedStr}
+                              </td>
+                              <td className="status-cell">
+                                <span className={`status-badge ${isStale ? 'stale' : 'running'}`}>
+                                  {isStale ? 'Stale?' : 'Running'}
+                                </span>
+                              </td>
+                            </tr>
+                          );
+                        })}
                       </tbody>
                     </table>
                   </div>