feat: bidirectional sync — upload local activities to remote instance

- Server: POST /api/upload/bas accepts pre-extracted BAS JSON (activity + optional timeseries/geojson), writes files and triggers merge_all
- sync.ts: uploadLocalActivities reads unsynced local activities by original_path, POSTs to /api/upload/bas, marks synced_at on success
- Settings: Upload toggle (Off / Upload local activities) in Sync section with subLabel dividers for Download / Upload groups
- Feed: sync message includes uploaded count when activities are pushed

mobile/app/(tabs)/index.tsx

@@ -20,12 +20,13 @@ export default function FeedScreen() {
       setSyncMsg(result.error);
     } else if (result.total === 0) {
       setSyncMsg('No activities on instance');
-    } else if (result.synced === 0 && !result.fetched) {
+    } else if (result.synced === 0 && !result.fetched && !result.uploaded) {
       setSyncMsg(`Up to date (${result.total} activities)`);
     } else {
       const parts = [];
       if (result.synced > 0) parts.push(`${result.synced} new`);
       if (result.fetched) parts.push(`${result.fetched} full dataset${result.fetched === 1 ? '' : 's'}`);
+      if (result.uploaded) parts.push(`${result.uploaded} uploaded`);
       setSyncMsg(`Synced: ${parts.join(', ')} (${result.total} total)`);
     }
     setTimeout(() => setSyncMsg(null), 3500);