feat(desktop): Tier A desktop features — web side (P5-46/36/44/43/49/47/55/57)

Web half of the desktop feature wave. A shared bridge (`hooks/useTauri.ts`:
invokeTauri/isTauri/useTauriEvent) backs per-feature hooks that no-op in the
browser and drive the native Tauri commands (compiled in cinny-desktop):

- P5-46 useTauriCallPower — hold system awake while a call is active.
- P5-36 useTauriJumpList — Windows jump list of recent rooms → matrix: deep links.
- P5-44 useTauriThumbbar — taskbar Mute/Deafen/End; events toggle mic/sound/hangup.
- P5-43 useTauriSmtc — SMTC call state + button events.
- P5-49 useTauriNetwork — react to native network-change → mx.retryImmediately().
- P5-47 window chrome — opt-in `customWindowChromeAtom` + TDS `TitleBar`; DesktopChrome
  wrapper in App.tsx (zero layout impact when off) + a desktop-only settings toggle.
- P5-55 composer toolbar drag-reorder (settings order[] + pragmatic-drag-and-drop).
- P5-57 DraftIndicator — subtle "draft saved" cue in the composer.

Client-scoped hooks mount via TauriDesktopFeatures in ClientNonUIFeatures; window
chrome mounts at App level. Gates: tsc/eslint/prettier clean, build OK, 556 tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/pages/App.tsx

@@ -1,4 +1,4 @@
-import React, { useEffect } from 'react';
+import React, { ReactNode, useEffect } from 'react';
 import { ErrorBoundary } from 'react-error-boundary';
 import { Provider as JotaiProvider, useAtomValue } from 'jotai';
 import {
@@ -25,6 +25,10 @@ import { useCompositionEndTracking } from '../hooks/useComposingCheck';
 import { settingsAtom } from '../state/settings';
 import { LotusToastContainer } from '../features/toast/LotusToastContainer';
 import { useTauriNotificationBadge } from '../hooks/useTauriNotificationBadge';
+import { useTauriWindowChrome } from '../hooks/useTauriWindowChrome';
+import { isTauri } from '../hooks/useTauri';
+import { TitleBar } from '../features/desktop/TitleBar';
+import { customWindowChromeAtom } from '../state/customWindowChrome';
 import { SeasonalEffect } from '../components/seasonal/SeasonalEffect';
 import { applyCustomAccent, removeCustomAccent } from '../utils/accentColor';
 import { zIndices } from '../styles/zIndex';
@@ -88,6 +92,23 @@ function TauriEffects() {
   return null;
 }
 
+// P5-47 — opt-in TDS window chrome. `useTauriWindowChrome` keeps the native OS
+// window decorations in sync with the setting; when a desktop user enables
+// custom chrome we replace the OS titlebar with <TitleBar/>. When off (the
+// default, and always in the browser) this returns children unchanged, so there
+// is zero layout impact for everyone else.
+function DesktopChrome({ children }: { children: ReactNode }) {
+  const customChrome = useAtomValue(customWindowChromeAtom);
+  useTauriWindowChrome();
+  if (!(isTauri() && customChrome)) return <>{children}</>;
+  return (
+    <div style={{ display: 'flex', flexDirection: 'column', height: '100vh' }}>
+      <TitleBar />
+      <div style={{ flexGrow: 1, minHeight: 0 }}>{children}</div>
+    </div>
+  );
+}
+
 function NightLightOverlay() {
   const settings = useAtomValue(settingsAtom);
   if (!settings.nightLightEnabled) return null;
@@ -160,7 +181,9 @@ function App() {
                         <JotaiProvider>
                           <AppearanceEffects />
                           <TauriEffects />
-                          <RouterProvider router={createRouter(clientConfig, screenSize)} />
+                          <DesktopChrome>
+                            <RouterProvider router={createRouter(clientConfig, screenSize)} />
+                          </DesktopChrome>
                           <SeasonalEffect />
                           <NightLightOverlay />
                           <LotusToastContainer />