fix(desktop): Custom Window Chrome toggle breaks the timeline (P5-47)

Toggling custom chrome expanded the screen and sent the message feed
auto-scrolling into the past. Two causes:
- DesktopChrome used height:100vh while html/#root use 100dvh; in the Tauri
  webview 100vh can exceed the visible height after decorations are stripped,
  making the timeline's scroll container taller than the viewport → the virtual
  paginator runs away paginating backwards. Switched to 100dvh.
- Toggling live reflowed the whole app while the timeline was mounted. The
  setting now persists + reloads so the layout is rebuilt cleanly (description
  updated: "reloads to apply").

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

src/app/pages/App.tsx

@@ -110,7 +110,11 @@ function DesktopChrome({ children }: { children: ReactNode }) {
     <div
       style={
         useChrome
-          ? { display: 'flex', flexDirection: 'column', height: '100vh' }
+          ? // Match html/#root (100dvh), NOT 100vh — in the Tauri webview 100vh
+            // can exceed the visible height after decorations are stripped, which
+            // makes the timeline's scroll container taller than the viewport and
+            // sends the virtual paginator into a runaway back-pagination loop.
+            { display: 'flex', flexDirection: 'column', height: '100dvh' }
           : { display: 'contents' }
       }
     >