feat(mobile): editable activity title for local activities

Adds edits_json column (migration v3) to store user overrides separately
from detail_json so Option A server re-extraction never clobbers them.

- Tap the title in the detail screen to edit (local activities only, shown
  with a ✎ hint). Saves on keyboard dismiss via onEndEditing.
- Cards and search display user_title ?? title.
- Raw upload: user_title sent to server -> sidecar written so web UI shows
  the correct title (server re-extracts from FIT, which has Karoo's title).
- BAS upload: detail.title overridden before sending, no sidecar needed.

mobile/components/ActivityCard.tsx

@@ -52,7 +52,7 @@ export function ActivityCard({
           }
         </View>
       </View>
-      <Text style={styles.cardTitle} numberOfLines={1}>{activity.title}</Text>
+      <Text style={styles.cardTitle} numberOfLines={1}>{activity.user_title ?? activity.title}</Text>
       <View style={styles.cardStats}>
         {km   && <Stat label="km" value={km} />}
         {elev != null && <Stat label="m↑" value={String(elev)} />}