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/pages/client/ClientNonUIFeatures.tsx

@@ -1015,13 +1015,13 @@ function TauriUpdateFeature() {
     dismissToast(UPDATE_PROGRESS_TOAST);
     if (!failure) return;
     // Short on purpose; the full explanation and a Download installer button
-    // are in Settings → General → App updates.
+    // are in Settings → Desktop → App updates.
     const busy = failure.phase !== 'install' && isNetworkUpdateError(failure.message);
     setToast({
       id: UPDATE_FAILED_TOAST,
       iconSrc: Icons.Warning,
       displayName: 'Update didn’t install',
-      body: `${busy ? 'The update server seems busy. ' : ''}Click to try again, or get the installer in Settings → General → App updates.`,
+      body: `${busy ? 'The update server seems busy. ' : ''}Click to try again, or get the installer in Settings → Desktop → App updates.`,
       roomName: 'System',
       roomId: '',
       sticky: true,
@@ -1062,7 +1062,7 @@ function TauriUpdateFeature() {
       roomName: 'System',
       roomId: '',
       onClick: () => {
-        if (viaPackage) requestSettings('general');
+        if (viaPackage) requestSettings('desktop');
         else installFromToast();
       },
       sticky: true,

src/app/pages/client/ClientRoot.tsx

@@ -61,7 +61,7 @@ import { AutoDiscovery } from './AutoDiscovery';
 
 // Capture-only E2EE diagnostics ring buffer (KE-1→4 signatures) — installed at
 // module load so it sees crypto warnings from the very first sync. Idempotent;
-// report download lives in Settings → Developer Tools → Crypto Diagnostics.
+// report download lives in Settings → Advanced → Crypto diagnostics.
 installCryptoDiagLog();
 
 function ClientRootLoading() {

src/app/pages/client/KeyUploadBanner.tsx

@@ -72,8 +72,8 @@ export function KeyUploadBanner() {
               rejects every new upload and other people can't start fresh encrypted sessions
               with you. This usually happens after the browser cleared this site's storage
               while you stayed signed in. Before signing out, make sure this device is verified or
-              key backup is on (Settings → Devices), so your message history comes back when you
-              sign in again.
+              key backup is on (Settings → Privacy & security), so your message history comes back
+              when you sign in again.
             </Text>
             {lastError && (
               <Text size="T200" align="Center" priority="300" style={{ wordBreak: 'break-word' }}>

src/app/pages/client/sidebar/SettingsTab.tsx

@@ -180,7 +180,10 @@ export function SettingsTab() {
   useEffect(() => {
     if (!settingsRequest) return;
     const page = {
-      general: SettingsPages.GeneralPage,
+      // [Gitea #459] General is gone; its old deep links land on Appearance.
+      general: SettingsPages.AppearancePage,
+      appearance: SettingsPages.AppearancePage,
+      desktop: SettingsPages.DesktopPage,
       account: SettingsPages.AccountPage,
       notifications: SettingsPages.NotificationPage,
       devices: SettingsPages.DevicesPage,