feat: task 2.7 event picker (subscription driver)

- src/data/events.ts: useUserEvents() TanStack Query (5-min stale,
  sort -starts_at). EventSummary type is a Pick of EventRow.
- src/live/active-event.ts: useActiveEventOrchestration() returns the
  swap fn — unsubscribe previous + clearForEvent + subscribe new +
  applySnapshot on success + persist to localStorage. Out-of-order
  safety via per-call version counter. Plus readSavedActiveEventId().
- src/ui/components/event-picker.tsx: <EventPicker> dropdown.
  useState + click-outside; rows show name + date + discipline.
- src/live/index.ts: re-exports active-event helpers.
- src/routes/_authed/monitor.tsx: auto-select effect (one-shot via
  initializedRef, gated on events loaded + WS connected); renders
  <EventPicker> wired to setActiveEvent.

Deviations:
1. Vanilla div + useState dropdown instead of shadcn Popover —
   no new shadcn primitive add; easy to swap later for keyboard nav.
2. Auto-select gated on connectionStatus === 'connected' so the
   subscribe call gets the snapshot path (not 'not-connected').
3. Logout-clears-saved-event-id deferred to a small Phase 1.8
   follow-up; documented in task risks.

Bundle: 395KB / 120KB gz (~1KB up from 2.6).

.planning/phase-2-live-map/README.md

@@ -54,7 +54,7 @@ When Phase 2 is done:
 | 2.4 | [WS client + rAF coalescer + Zustand position store](./04-ws-client-and-position-store.md) | 🟩     |
 | 2.5 | [MapPositions (clustered + selected sources)](./05-map-positions.md)                       | 🟩     |
 | 2.6 | [MapTrails (bounded ring buffer, polyline rendering)](./06-map-trails.md)                  | 🟩     |
-| 2.7 | [Event picker (subscription driver)](./07-event-picker.md)                                 | ⬜     |
+| 2.7 | [Event picker (subscription driver)](./07-event-picker.md)                                 | 🟩     |
 | 2.8 | [Camera control trio](./08-camera-trio.md)                                                 | ⬜     |
 | 2.9 | [Connection status + per-device last-seen indicators](./09-connection-status.md)           | ⬜     |