feat(settings): quick wins from the settings audit (#453)

- #456 Copy pass: sentence case for every setting title and section,
  plainer names ("Hide joins and leaves", "Link previews", "Emoji
  style: Twemoji", "Rejoin calls after a restart", "Also use the
  browser's noise suppression", "Push-to-talk key"...) and grammar fixes
  ("when messages arrive", "Play a sound when a new message arrives",
  blocked users "Enter their Matrix ID"). Labels only: no setting is
  renamed in storage.
- #455 One theme control: Theme: System / Light / Dark. System shows the
  light and dark variant pickers, Light or Dark one picker of that kind.
  Replaces the System switch plus a Theme dropdown that was greyed out
  whenever the switch was on. Uses the existing settings (no migration);
  picking a variant also remembers it for System.
- #457 Remove the "Block Messages" stub from Notifications; the GIF
  picker row only appears in builds with a GIF key.
- #458 (part 1) "Export/Import Messages Data" -> "Export/Import
  encryption keys", with a sentence on when you need it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/pages/client/ClientNonUIFeatures.tsx

@@ -1015,13 +1015,13 @@ function TauriUpdateFeature() {
     dismissToast(UPDATE_PROGRESS_TOAST);
     if (!failure) return;
     // Short on purpose; the full explanation and a Download installer button
-    // are in Settings → General → App Updates.
+    // are in Settings → General → App updates.
     const busy = failure.phase !== 'install' && isNetworkUpdateError(failure.message);
     setToast({
       id: UPDATE_FAILED_TOAST,
       iconSrc: Icons.Warning,
       displayName: 'Update didn’t install',
-      body: `${busy ? 'The update server seems busy. ' : ''}Click to try again, or get the installer in Settings → General → App Updates.`,
+      body: `${busy ? 'The update server seems busy. ' : ''}Click to try again, or get the installer in Settings → General → App updates.`,
       roomName: 'System',
       roomId: '',
       sticky: true,