Email confirmation, change email/password, show/hide passwords

- Sign-up emails a confirmation link (24h); unconfirmed accounts see a
  banner on the web with "send it again", and status in Settings.
- Change email (needs the password): the new address gets a confirmation
  link and only replaces the old one once clicked; the old address gets a
  heads-up with a reset-password link. Pending changes can be cancelled.
- Change password (needs the current one): signs out other sessions.
- Password fields have Show/Hide on web and mobile.

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

apps/mobile/src/app/_layout.tsx

@@ -96,6 +96,8 @@ function AppNavigator() {
           <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="change-password" options={{ title: 'Change password', presentation: 'modal' }} />
+          <Stack.Screen name="change-email" options={{ title: 'Change email', presentation: 'modal' }} />
           <Stack.Screen name="scan" options={{ title: 'Scan a code', presentation: 'modal' }} />
         </Stack.Protected>
         <Stack.Protected guard={!signedIn}>