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/utils/securityNudge.ts

@@ -10,7 +10,7 @@
  *   cross-signing: the setup flow does both).
  *
  * - stale-sessions (#113, lowest priority): sessions of this account not used
- *   for 90+ days; Review opens Settings → Devices with them ticked to sign out.
+ *   for 90+ days; Review opens Settings → Privacy & security with them ticked to sign out.
  *
  * Anything not yet known (crypto still loading) → no nudge. Timing: not in
  * the first 24 h on a device (onboarding), "Not now" snoozes 7 days, three