feat(settings): split General into pages; collapse the big pickers (#459, #460)

#459 General held 76 of ~110 settings (11 screens). Its sections now live
on pages named for what people come to do:
  Account, Appearance, Messages (incl. editor, date and time),
  Notifications, Calls & audio, Privacy & security (privacy options +
  the former Devices page), Data & storage (storage, sync, your data),
  Desktop (desktop app only), Emojis & stickers, Advanced (the former
  Developer Tools + "show hidden events" / "legacy username colour"),
  About.
The longest page is now ~3 screens (Appearance). Settings opens on
Account. Deep links keep working: "devices" lands on Privacy & security,
the old "general" on Appearance, the update notice on Desktop. Text that
pointed at the old page names is updated. The push-rule editor gets one
line explaining the Matrix terms. Remaining Title Case labels on Account
and Devices are sentence case.

#460 Chat background, seasonal theme and avatar decoration grids sit
behind one row: the current choice and a Change button (the first two
close after a choice; decorations keep their Save step).

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

src/app/components/CloseBehaviorPrompt.tsx

@@ -19,7 +19,7 @@ import { useModalStyle } from '../hooks/useModalStyle';
  * cinny-desktop #5: the first time the window is closed, ask whether Lotus Chat
  * should keep running in the tray (calls, messages, notifications) or quit —
  * and, in the same moment, whether it should start when you sign in. Asked
- * once; changeable in Settings → General. The native side sends
+ * once; changeable in Settings → Desktop. The native side sends
  * `lotus-close-requested` only while the choice is still "ask", and never
  * during a call (a call always goes to the tray).
  */
@@ -149,7 +149,7 @@ export function CloseBehaviorPrompt() {
                 )}
               </Box>
               <Text size="T200" priority="300">
-                You can change this in Settings → General.
+                You can change this in Settings → Desktop.
               </Text>
             </Box>
           </Dialog>