feat(workers): Add platform badge (D/J/T) to active tasks display

- Add PlatformBadge component showing D=Dutchie, J=Jane, T=Treez
- Include platform field in worker-registry API response
- Fix null running_seconds displaying as "nulls"

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/routes/worker-registry.ts

@@ -436,6 +436,7 @@ router.get('/workers', async (req: Request, res: Response) => {
         t.worker_id,
         t.id as task_id,
         t.role,
+        t.platform,
         t.status as task_status,
         t.started_at,
         EXTRACT(EPOCH FROM (NOW() - t.started_at))::int as running_seconds,
@@ -458,6 +459,7 @@ router.get('/workers', async (req: Request, res: Response) => {
       tasksByWorker[task.worker_id].push({
         task_id: task.task_id,
         role: task.role,
+        platform: task.platform,
         status: task.task_status,
         started_at: task.started_at,
         running_seconds: task.running_seconds,