feat: bulk operations in Saved tab (export, upload, delete, merge)

Select button in header enters selection mode. Cards show a checkbox and
become tappable. Header updates to show N selected + Cancel.

Bulk actions (bottom bar):
- Export: sequential Sharing.shareAsync for each selected file
- Upload: sequential upload with N/total progress text
- Delete: confirm then delete all selected at once
- Merge: parses each GPX file, combines track points sorted by time,
  prompts for title via Modal, saves as new recording

parseGpxFile() added to gpx.ts: reads file via expo-file-system,
extracts trkpt elements including hr/power/cad extensions via regex.

src/services/gpx.ts

@@ -55,6 +55,27 @@ function buildExtensions(pt: TrackPoint): string {
         </extensions>`;
 }
 
+export async function parseGpxFile(fileUri: string): Promise<TrackPoint[]> {
+  const content = await new File(fileUri).text();
+  const points: TrackPoint[] = [];
+  const re = /<trkpt\s+lat="([^"]+)"\s+lon="([^"]+)">([\s\S]*?)<\/trkpt>/g;
+  let m;
+  while ((m = re.exec(content)) !== null) {
+    const lat = parseFloat(m[1]);
+    const lon = parseFloat(m[2]);
+    const inner = m[3];
+    const ele     = parseFloat(inner.match(/<ele>([\d.-]+)<\/ele>/)?.[1] ?? '0');
+    const timeStr = inner.match(/<time>([^<]+)<\/time>/)?.[1] ?? '';
+    const hr      = parseInt(inner.match(/<gpxtpx:hr>(\d+)<\/gpxtpx:hr>/)?.[1] ?? '') || undefined;
+    const power   = parseInt(inner.match(/<gpxtpx:power>(\d+)<\/gpxtpx:power>/)?.[1] ?? '') || undefined;
+    const cad     = parseInt(inner.match(/<gpxtpx:cad>(\d+)<\/gpxtpx:cad>/)?.[1] ?? '') || undefined;
+    if (!isNaN(lat) && !isNaN(lon)) {
+      points.push({ lat, lon, ele, time: new Date(timeStr), hr, power, cad });
+    }
+  }
+  return points;
+}
+
 function escapeXml(s: string): string {
   return s.replace(/&/g, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;').replace(/"/g, '&quot;');
 }