refactor: extract device detection into hook and localize PWA handling to Sidebar

- Add new `useDeviceSettings` hook (`src/hooks/useDeviceSettings.ts`) to centralize
  device-related state:
  - exposes `isMobile` and `isPWA`
  - supports options: `mobileBreakpoint`, `trackMobile`, `trackPWA`
  - listens to window resize for mobile updates
  - listens to `display-mode: standalone` changes for PWA updates
  - includes `matchMedia.addListener/removeListener` fallback for older environments

- Update `AppContent` (`src/App.jsx`) to consume `isMobile` from
  `useDeviceSettings({ trackPWA: false })`:
  - remove local `isMobile` state/effect
  - remove local `isPWA` state/effect
  - keep existing `isMobile` behavior for layout and mobile sidebar flow
  - stop passing `isPWA` into `Sidebar` props

- Update `Sidebar` (`src/components/Sidebar.jsx`) to own PWA detection:
  - consume `isPWA` from `useDeviceSettings({ trackMobile: false })`
  - add effect to toggle `pwa-mode` class on `document.documentElement` and `document.body`
  - retain use of `isMobile` prop from `App` for sidebar/mobile rendering decisions

Why:
- removes duplicated device-detection logic from `AppContent`
- makes device-state logic reusable and easier to maintain
- keeps PWA-specific behavior where it is actually used (`Sidebar`)

src/components/Sidebar.jsx

@@ -14,6 +14,7 @@ import CodexLogo from './CodexLogo.jsx';
 import TaskIndicator from './TaskIndicator';
 import ProjectCreationWizard from './ProjectCreationWizard';
 import VersionUpgradeModal from './modals/VersionUpgradeModal';
+import { useDeviceSettings } from '../hooks/useDeviceSettings';
 import { useVersionCheck } from '../hooks/useVersionCheck';
 import { useUiPreferences } from '../hooks/useUiPreferences';
 import { api } from '../utils/api';
@@ -36,10 +37,10 @@ function Sidebar({
   loadingProgress,
   onRefresh,
   onShowSettings,
-  isPWA,
   isMobile
 }) {
   const { t } = useTranslation(['sidebar', 'common']);
+  const { isPWA } = useDeviceSettings({ trackMobile: false });
   const { updateAvailable, latestVersion, currentVersion, releaseInfo } = useVersionCheck('siteboon', 'claudecodeui');
   const { preferences, setPreference } = useUiPreferences();
   const { sidebarVisible } = preferences;
@@ -92,6 +93,15 @@ function Sidebar({
     };
   };
 
+  useEffect(() => {
+    if (typeof document === 'undefined') {
+      return;
+    }
+
+    document.documentElement.classList.toggle('pwa-mode', isPWA);
+    document.body.classList.toggle('pwa-mode', isPWA);
+  }, [isPWA]);
+
   // Auto-update timestamps every minute
   useEffect(() => {
     const timer = setInterval(() => {