Safety screens: followers, blocking, reporting, admin

Web and mobile:
- Follower/following lists (tap the counts), with Follow buttons and
  "Remove" on your own followers.
- Profile ⋯ menu: block (with confirmation) / unblock, and report with a
  reason and optional note. "Follows you" label, "Follow back", and a
  banner on profiles you've blocked.
- Blocked accounts list in Settings, with unblock.
Web only: /admin for the admin account: stats, open/resolved reports,
resolve, suspend/unsuspend.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MzFywujuH9TiuXbr9vjoX8

apps/mobile/src/app/_layout.tsx

@@ -91,7 +91,11 @@ function AppNavigator() {
         </Stack.Protected>
         <Stack.Protected guard={signedIn}>
           {/* Reachable from onboarding too (scanning a friend's code, viewing a profile). */}
-          <Stack.Screen name="u/[username]" options={{ title: '' }} />
+          <Stack.Screen name="u/[username]/index" options={{ title: '' }} />
+          <Stack.Screen name="u/[username]/followers" options={{ title: 'Followers' }} />
+          <Stack.Screen name="u/[username]/following" options={{ title: 'Following' }} />
+          <Stack.Screen name="report/[username]" options={{ title: 'Report', presentation: 'modal' }} />
+          <Stack.Screen name="blocked" options={{ title: 'Blocked accounts' }} />
           <Stack.Screen name="scan" options={{ title: 'Scan a code', presentation: 'modal' }} />
         </Stack.Protected>
         <Stack.Protected guard={!signedIn}>