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.

src/routes/_authed/monitor.tsx

@@ -1,4 +1,5 @@
 import { createFileRoute } from '@tanstack/react-router';
+import { BasemapSwitcher } from '@/map/core/basemap-switcher';
 import { MapView } from '@/map/core/map-view';
 
 export const Route = createFileRoute('/_authed/monitor')({
@@ -11,7 +12,17 @@ function MonitorPage() {
     // For now the parent _authed layout has no top-bar, so the map can
     // fill the full viewport — adjust when chrome lands.
     <div className="relative h-[calc(100vh-0rem)] w-full">
-      <MapView />
+      <MapView>
+        {/*
+          BasemapSwitcher renders inside <MapView> so it remounts after
+          every style swap, ensuring the persisted preference is re-applied
+          if the user does something funny (e.g. force-reloads style via
+          devtools). It's also a sibling of the map canvas, not on top of
+          it visually — the absolute-positioned card is positioned relative
+          to <MapView>'s wrapper.
+        */}
+        <BasemapSwitcher />
+      </MapView>
     </div>
   );
 }