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);

mobile/app/(tabs)/settings.tsx

@@ -13,7 +13,8 @@ export default function SettingsScreen() {
   const storedHandle   = useSetting('handle') ?? '';
   const storedPath     = useSetting('auto_import_path') ?? '';
   const storedToken    = useSetting('api_token');
-  const storedSyncMode = (useSetting('sync_mode') ?? 'summaries') as 'summaries' | 'full';
+  const storedSyncMode   = (useSetting('sync_mode') ?? 'summaries') as 'summaries' | 'full';
+  const storedSyncUpload = useSetting('sync_upload') === 'true';
 
   const [instanceUrl, setInstanceUrl] = useState(storedUrl);
   const [handle,      setHandle]      = useState(storedHandle);
@@ -176,6 +177,7 @@ export default function SettingsScreen() {
       )}
 
       <Section title="Sync">
+        <Text style={styles.subLabel}>Download</Text>
         <View style={styles.modeRow}>
           <ModeButton
             label="Summaries only"
@@ -193,6 +195,24 @@ export default function SettingsScreen() {
             ? 'Downloads map route and elevation chart for every activity during sync. Uses more storage and takes longer.'
             : 'Syncs activity summaries only. Map and chart are fetched on demand when you open an activity.'}
         </Text>
+        <Text style={[styles.subLabel, { borderTopWidth: 1, borderTopColor: '#27272a', paddingTop: 12 }]}>Upload</Text>
+        <View style={styles.modeRow}>
+          <ModeButton
+            label="Off"
+            active={!storedSyncUpload}
+            onPress={() => setSetting(db, 'sync_upload', 'false')}
+          />
+          <ModeButton
+            label="Upload local activities"
+            active={storedSyncUpload}
+            onPress={() => setSetting(db, 'sync_upload', 'true')}
+          />
+        </View>
+        <Text style={styles.hint}>
+          {storedSyncUpload
+            ? 'Local activities are uploaded to the instance during sync.'
+            : 'Local activities stay on device only.'}
+        </Text>
       </Section>
 
       <Section title="Data">
@@ -314,6 +334,7 @@ const styles = StyleSheet.create({
   disconnectText: { color: '#71717a', fontSize: 14 },
   msgOk:  { color: '#86efac', fontSize: 13, paddingHorizontal: 12, paddingBottom: 10 },
   msgErr: { color: '#fca5a5', fontSize: 13, paddingHorizontal: 12, paddingBottom: 10 },
+  subLabel:           { color: '#52525b', fontSize: 11, fontWeight: '600', letterSpacing: 0.6, paddingHorizontal: 12, paddingTop: 12, paddingBottom: 4 },
   modeRow:            { flexDirection: 'row', gap: 8, padding: 12 },
   modeButton:         { flex: 1, paddingVertical: 9, borderRadius: 8, borderWidth: 1, borderColor: '#3f3f46', alignItems: 'center' },
   modeButtonActive:   { backgroundColor: '#1e3a5f', borderColor: '#2563eb' },