feat: offline map download via MapLibre OfflineManager

New src/services/offline.ts:
- downloadRegion(): createPack with bounds, zoom 6-16, progress callback
- listRegions() / deleteRegion(): pack management
- expandBounds(): adds 5km buffer around the visible area
- formatBytes(): human-readable size string

RecordingScreen:
- MapRef attached to Map component to read getBounds()
- '↓ Offline' overlay button (Liberty style + idle state only)
- Modal: name input → download → progress bar with % and MB counter
- Raster styles show no download button (not supported by OfflineManager)

Settings → App → Offline maps:
- Lists all downloaded regions with size and tile count
- Delete with confirm alert
- Placeholder text when no regions exist

src/services/offline.ts

@@ -0,0 +1,100 @@
+import { OfflineManager, type OfflinePackStatus } from '@maplibre/maplibre-react-native';
+
+type LngLatBounds = [number, number, number, number]; // [west, south, east, north]
+
+// Vector style URL — only vector styles support offline packs
+export const OFFLINE_STYLE_URL = 'https://tiles.openfreemap.org/styles/liberty';
+
+// Zoom levels: 6 (region overview) → 16 (street detail, good for cycling)
+const MIN_ZOOM = 6;
+const MAX_ZOOM = 16;
+
+export interface OfflineRegion {
+  id: string;
+  name: string;
+  bounds: LngLatBounds;
+  createdAt: string;
+  completedTiles: number;
+  totalSizeBytes: number;
+  state: 'active' | 'inactive' | 'complete';
+  percentage: number;
+}
+
+// ── Download ──────────────────────────────────────────────────────────────────
+
+export async function downloadRegion(
+  name: string,
+  bounds: LngLatBounds,
+  onProgress: (pct: number, tilesDown: number, sizeBytes: number) => void,
+  onError: (msg: string) => void,
+): Promise<string> {
+  OfflineManager.setProgressEventThrottle(500);
+
+  const pack = await OfflineManager.createPack(
+    {
+      mapStyle: OFFLINE_STYLE_URL,
+      bounds,
+      minZoom: MIN_ZOOM,
+      maxZoom: MAX_ZOOM,
+      metadata: { name, createdAt: new Date().toISOString() },
+    },
+    (_pack, status: OfflinePackStatus) => {
+      onProgress(status.percentage, status.completedTileCount, status.completedTileSize);
+    },
+    (_pack, error) => {
+      onError(error.message);
+    },
+  );
+
+  return pack.id;
+}
+
+// ── List ──────────────────────────────────────────────────────────────────────
+
+export async function listRegions(): Promise<OfflineRegion[]> {
+  const packs = await OfflineManager.getPacks();
+  const regions: OfflineRegion[] = [];
+
+  for (const pack of packs) {
+    const status = await pack.status();
+    const meta = pack.metadata as { name?: string; createdAt?: string };
+    regions.push({
+      id: pack.id,
+      name: meta.name ?? 'Unnamed region',
+      bounds: pack.bounds,
+      createdAt: meta.createdAt ?? '',
+      completedTiles: status.completedTileCount,
+      totalSizeBytes: status.completedTileSize,
+      state: status.state,
+      percentage: status.percentage,
+    });
+  }
+
+  return regions;
+}
+
+// ── Delete ────────────────────────────────────────────────────────────────────
+
+export async function deleteRegion(id: string): Promise<void> {
+  await OfflineManager.deletePack(id);
+}
+
+// ── Helpers ───────────────────────────────────────────────────────────────────
+
+export function formatBytes(bytes: number): string {
+  if (bytes < 1024) return `${bytes} B`;
+  if (bytes < 1024 * 1024) return `${(bytes / 1024).toFixed(1)} KB`;
+  return `${(bytes / (1024 * 1024)).toFixed(1)} MB`;
+}
+
+/** Expand a [w,s,e,n] bounds by `km` kilometres in every direction. */
+export function expandBounds(bounds: LngLatBounds, km: number): LngLatBounds {
+  const latDelta = km / 111;
+  const lonDelta = km / (111 * Math.cos((bounds[1] + bounds[3]) / 2 * Math.PI / 180));
+  return [
+    bounds[0] - lonDelta,
+    bounds[1] - latDelta,
+    bounds[2] + lonDelta,
+    bounds[3] + latDelta,
+  ];
+}