fix(windows): eliminate startup white-black flash (FOUC)

Set window backgroundColor to match the dark theme surface so WebView2's
native window layer is dark before any web content paints. Add an inline
anti-FOUC script in index.html to apply the theme class synchronously
before first paint, reading from localStorage with system fallback.
Update CSP script-src to allow the inline script via SHA-256 base64 hash
alongside the existing 'self' source.

Fixes #6182

src-tauri/tauri.conf.json

@@ -20,13 +20,14 @@
         "minWidth": 900,
         "minHeight": 600,
         "visible": false,
+        "backgroundColor": "#1e1e22",
         "resizable": true,
         "fullscreen": false,
         "center": true
       }
     ],
     "security": {
-      "csp": "default-src 'self'; img-src 'self' data: https: http:; script-src 'self'; style-src 'self' 'unsafe-inline'; connect-src 'self' ipc: http://ipc.localhost https: http:",
+      "csp": "default-src 'self'; img-src 'self' data: https: http:; script-src 'self' 'sha256-ls27Lc1OytcVawXPVMplz6LyKz2i9KYAVBw3u79qyB4='; style-src 'self' 'unsafe-inline'; connect-src 'self' ipc: http://ipc.localhost https: http:",
       "assetProtocol": {
         "enable": true,
         "scope": []

src/index.html

@@ -4,6 +4,7 @@
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <title>Claude Code 供应商切换器</title>
+    <script>(function(){try{var t=localStorage.getItem("cc-switch-theme")||"system";var d=t==="dark"||(t==="system"&&matchMedia("(prefers-color-scheme: dark)").matches);document.documentElement.classList.toggle("dark",d);}catch(e){}})()</script>
   </head>
   <body>
     <div id="root"></div>