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

e2e/security-nudge.spec.ts

@@ -83,7 +83,7 @@ test.describe('security nudge (#110, #123)', () => {
     const nudge = strip(page);
     await expect(nudge).toContainText("Your encryption keys aren't backed up");
     await nudge.getByRole('button', { name: 'Set up' }).click();
-    await expect(page.getByText('Device Verification').first()).toBeVisible();
+    await expect(page.getByText('Device verification').first()).toBeVisible();
   });
 
   test('a dismissed announcement no longer hides the nudge (#307)', async ({ page }) => {