feat: task 2.8 camera control trio + follow toggle

- src/map/core/map-pref-store.ts: mapFollow boolean (default true) +
  setter. Persisted via trm-map-prefs.
- src/map/core/camera/default-camera.tsx: <MapDefaultCamera /> fits
  the snapshot's bounds once per activeEventId change. Uses a
  fittedFor ref; resets to null when activeEventId becomes null.
  Single-point events centre+zoom 12; multi-point fitBounds with
  padding capped at min(canvas*0.1, 80).
- src/map/core/camera/selected-device.tsx: <MapSelectedDevice />
  pans+zooms on selection change, smooth pans on subsequent position
  updates with mapFollow on. Separate effect listens for user-gesture
  movestart (originalEvent truthy) and flips mapFollow off.
- src/map/core/camera/map-camera.tsx: <MapCamera coordinates> imperative
  one-shot. Phase 2 doesn't use it; primitive for replay (Phase 4) and
  future "fit to all" UI.
- src/map/core/follow-toggle.tsx: <FollowToggle /> icon-button using
  Lucide Locate/LocateOff. Sits below the trails toggle.
- src/routes/_authed/monitor.tsx: renders FollowToggle, MapDefaultCamera,
  MapSelectedDevice.

Deviations:
- Manual-pan listener uses an inline { originalEvent?: unknown } type
  instead of MapLibre's MapMouseEvent|MapTouchEvent union (typing
  friction across version bumps).
- MapDefaultCamera clears fittedFor on activeEventId === null so
  re-selecting the same event later re-fits.

Bundle: main 395KB / 120KB gz, no measurable change.

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

@@ -55,7 +55,7 @@ When Phase 2 is done:
 | 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)                                 | 🟩     |
-| 2.8 | [Camera control trio](./08-camera-trio.md)                                                 | ⬜     |
+| 2.8 | [Camera control trio](./08-camera-trio.md)                                                 | 🟩     |
 | 2.9 | [Connection status + per-device last-seen indicators](./09-connection-status.md)           | ⬜     |
 
 ## Files modified