feat: task 2.6 MapTrails (bounded ring buffer, polyline rendering)

- src/map/core/map-pref-store.ts: trailMode preference
  ('none' | 'selected' | 'all', default 'selected') + setter, persisted.
- src/map/layers/map-trails.tsx: <MapTrails /> side-effect-only.
  Single GeoJSON source + single line layer. Builds one LineString
  Feature per device whose trail has >= 2 points; filtered by mode.
  Per-device flat colour via deterministic deviceId hash mod a
  6-entry palette.
- src/map/core/trails-toggle.tsx: <TrailsToggle /> floating card
  below <BasemapSwitcher />. Three buttons (None / Selected / All).
- src/routes/_authed/monitor.tsx: renders <TrailsToggle /> +
  <MapTrails /> *before* <MapPositions /> so the line layer is added
  to the map first and renders underneath the symbol layers.

Speed-coloured-per-segment deferred to Phase 3 polish per the task
spec's open-question decision; flat-colour-per-device for v1.

Bundle: main 394KB / 120KB gz — no change from 2.5.

src/routes/_authed/monitor.tsx

@@ -1,7 +1,9 @@
 import { createFileRoute } from '@tanstack/react-router';
 import { BasemapSwitcher } from '@/map/core/basemap-switcher';
 import { MapView } from '@/map/core/map-view';
+import { TrailsToggle } from '@/map/core/trails-toggle';
 import { MapPositions } from '@/map/layers/map-positions';
+import { MapTrails } from '@/map/layers/map-trails';
 
 export const Route = createFileRoute('/_authed/monitor')({
   component: MonitorPage,
@@ -15,14 +17,17 @@ function MonitorPage() {
     <div className="relative h-[calc(100vh-0rem)] w-full">
       <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.
+          UI controls. Floating cards positioned absolutely inside
+          <MapView>'s wrapper.
         */}
         <BasemapSwitcher />
+        <TrailsToggle />
+        {/*
+          Layer order matters — MapLibre paints in addLayer() call order.
+          <MapTrails /> mounts first so its line layer renders below the
+          symbol layers added by <MapPositions />.
+        */}
+        <MapTrails />
         <MapPositions />
       </MapView>
     </div>