feat: task 2.2 tile-source switcher

- pnpm add maplibre-google-maps (runtime; lazy-imported on Google use).
- src/map/core/styles.ts: BasemapDescriptor model with four entries
  (Esri Satellite / OpenTopoMap / OSM / Google Satellite).
  Google entry gated on cfg.googleMapsKey via available(cfg).
  styleCustom() builds a single-raster-source MapLibre style with a
  glyphs URL for future label rendering.
  ensureGoogleProtocol() lazy-loads maplibre-google-maps + addProtocol
  on first use; _googleRegistered guards re-registration.
- src/map/core/map-pref-store.ts: Zustand + persist middleware
  (key: trm-map-prefs, version: 1). Holds basemapId + setBasemap.
- src/map/core/basemap-switcher.tsx: floating control top-right of
  <MapView>. Filters by available(cfg), highlights active, applies via
  getMap().setStyle(). Mount-time bootstrap (no UI state) + user-driven
  onClick (with switching state) split to satisfy React 19's
  react-hooks/set-state-in-effect rule.
- src/routes/_authed/monitor.tsx: renders <BasemapSwitcher /> as a
  child of <MapView>.
- src/vite-env.d.ts: ambient module declaration for
  maplibre-google-maps (no .d.ts ships in the package).

Deviations:
- BasemapDescriptor.style is buildStyle(cfg): StyleSpecification, not
  the spec's StyleSpecification|string union. Google needs the cfg key
  at build time; uniform function shape across all entries is cleaner.
- Bootstrap apply path doesn't use the switching UI state (no visual
  feedback needed during the silent first-paint flip from OSM to user
  preference). User-click path keeps setSwitching for the in-flight
  indicator.

Bundle: MapLibre is now its own chunk (1MB / 273KB gz), shared across
maplibre consumers. Main bundle 371KB / 114KB gz.

.planning/phase-2-live-map/01-mapview-singleton.md

@@ -203,12 +203,14 @@ Three new files + small updates to existing surfaces:
 **Deviation flagged:**
 
 The spec sketched the singleton's accessor as `map` (a top-level export). Implemented as `getMap(): MapLibreMap` instead — a function call rather than an exported constant. Two reasons:
+
 1. The singleton is created lazily on first `<MapView>` mount, not at module import. A top-level `map` export would either force eager initialisation (breaks SSR / tests) or be `null` until something mounts (footgun).
 2. `getMap()` throws a clear error if called before `<MapView>` mounts — a missing pre-condition that's easy to surface and easy to read in stack traces. A `null`-able export forces every consumer to add their own null check.
 
-Side-effect-only `Map*` components (2.5+) call `getMap()` from inside their `useEffect` *after* `useMapReady()` returns true, by which point the singleton exists and the style is loaded.
+Side-effect-only `Map*` components (2.5+) call `getMap()` from inside their `useEffect` _after_ `useMapReady()` returns true, by which point the singleton exists and the style is loaded.
 
 **Smoke check (local `pnpm dev`):**
+
 - `/monitor` renders a full-viewport OSM map.
 - Pan / zoom / scroll-zoom all work.
 - `/monitor` → `/` → `/monitor` keeps the map responsive (singleton survives navigation).