feat: task management enhancements (cancelled status, recurrence, timestamps)

- Add 'cancelled' status to TaskStatus type, StatusBadge, TaskCard,
  TaskEditorView, TaskViewerView, TasksListView
- Add RecurrenceEditor component (none / interval / calendar rules)
- TaskEditorView: wire RecurrenceEditor, show started_at/completed_at
  timestamps read-only, include recurrence_rule in save payload
- TaskViewerView: show recurrence summary, timestamps in meta row
- tasks.ts: statusFilter/priorityFilter as arrays, add recurrence_rule
  to updateTask and createTask payloads

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

frontend/src/components/StatusBadge.vue

@@ -12,6 +12,7 @@ const labels: Record<TaskStatus, string> = {
   todo: "Todo",
   in_progress: "In Progress",
   done: "Done",
+  cancelled: "Cancelled",
 };
 </script>
 
@@ -48,6 +49,10 @@ const labels: Record<TaskStatus, string> = {
   background: color-mix(in srgb, var(--color-status-done-bg) 78%, var(--color-status-done) 22%);
   color: color-mix(in srgb, var(--color-status-done) 85%, #000 15%);
 }
+.status-cancelled {
+  background: color-mix(in srgb, var(--color-bg-secondary) 78%, var(--color-text-muted) 22%);
+  color: var(--color-text-muted);
+}
 .clickable {
   cursor: pointer;
 }