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/hooks/useSettingsSync.ts

@@ -140,7 +140,7 @@ export function useSettingsSync(): void {
 }
 
 /**
- * Manual actions for Settings → General → Sync. `pushNow` stamps a fresh
+ * Manual actions for Settings → Data & storage → Sync. `pushNow` stamps a fresh
  * snapshot of THIS device so it wins on every other device; `clearRemote`
  * blanks the account-data event (other devices ignore the invalid content and
  * keep their local settings until someone pushes again).