fix: restore is_no_compositing_mode useEffect for Linux frost fallback

The useEffect that invokes is_no_compositing_mode was removed along
with StartupShell, but it is the sole trigger for #553/#570's CSS
frost fallback on Linux. Without it, WEBKIT_DISABLE_COMPOSITING_MODE=1
users never get the fallback and see broken frosted glass.

Also fix the dead ternary: isTauri ? 'ready' : 'ready' → 'ready'.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

openless-all/app/src/App.tsx

@@ -17,6 +17,7 @@ import {
   windowMouseHotkeyCode,
 } from './lib/windowHotkeyFallback';
 import { QaPanel } from './pages/QaPanel';
+import { invoke } from '@tauri-apps/api/core';
 import { HotkeySettingsProvider } from './state/HotkeySettingsContext';
 
 interface AppProps {
@@ -37,7 +38,7 @@ export function App({ isCapsule, isQa, forcedOs }: AppProps) {
 
   const os = forcedOs ?? detectOS();
   // Windows 启动不应被权限探测阻塞首屏。
-  const [gate, setGate] = useState<Gate>(isTauri ? 'ready' : 'ready');
+  const [gate, setGate] = useState<Gate>('ready');
 
   useEffect(() => {
     if (!isTauri) return;
@@ -168,6 +169,18 @@ export function App({ isCapsule, isQa, forcedOs }: AppProps) {
     };
   }, [os]);
 
+  // Linux: 检测 WEBKIT_DISABLE_COMPOSITING_MODE → 触发磨砂 fallback（#570, #553）
+  useEffect(() => {
+    if (!isTauri) return;
+    invoke<boolean>('is_no_compositing_mode').then((val) => {
+      if (val) {
+        document.documentElement.dataset.olNoCompositing = 'true';
+      }
+    }).catch((err) => {
+      console.warn('[startup] is_no_compositing_mode failed', err);
+    });
+  }, []);
+
   return (
     <HotkeySettingsProvider>
       {gate === 'onboarding' ? <Onboarding onComplete={() => setGate('ready')} /> : <FloatingShell os={os} />}