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/features/room/thread/ThreadPanel.tsx

@@ -128,7 +128,7 @@ export function ThreadPanel({ room, threadId, requestClose }: ThreadPanelProps)
   const editor = useEditor();
   const thread = useThreadInstance(room, threadId);
   const [privateReadReceipts] = useSetting(settingsAtom, 'privateReadReceipts');
-  // "Hide Typing & Read Receipts" must also make thread receipts private (matches markAsRead).
+  // "Hide typing and read receipts" must also make thread receipts private (matches markAsRead).
   const [hideActivity] = useSetting(settingsAtom, 'hideActivity');
   const fileDropContainerRef = useRef<HTMLDivElement>(null) as React.RefObject<HTMLDivElement>;