fix: 沙盒页英文换行 + Logo 多语言

- 沙盒页标题/副标题添加 truncate 防止英文换行
- Logo 使用 t.brand.name/suffix 实现中英切换（宇之然/Yuzhiran）
- 新增 brand 翻译键（zh: 宇之然, en: Yuzhiran）

frontend/src/app/sandbox/page.tsx

@@ -306,20 +306,20 @@ function SandboxPage() {
 
   return (
     <div className="max-w-6xl mx-auto px-4 sm:px-6 lg:px-8 py-8">
-      <div className="flex items-center justify-between mb-4">
-        <div className="flex items-center gap-3">
+      <div className="flex items-center justify-between mb-4 gap-3">
+        <div className="flex items-center gap-3 min-w-0 flex-shrink overflow-hidden">
           <button onClick={() => setSessionsOpen(!sessionsOpen)}
-            className="lg:hidden p-2 text-muted-foreground hover:text-foreground rounded-lg hover:bg-accent">
+            className="lg:hidden p-2 text-muted-foreground hover:text-foreground rounded-lg hover:bg-accent flex-shrink-0">
             <svg className="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
               <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M4 6h16M4 12h16M4 18h16" />
             </svg>
           </button>
-          <div>
-            <h1 className="text-2xl font-bold text-foreground">{t.sandbox.title}</h1>
-            <p className="text-sm text-muted-foreground mt-0.5">{t.sandbox.subtitle}</p>
+          <div className="truncate">
+            <h1 className="text-2xl font-bold text-foreground truncate">{t.sandbox.title}</h1>
+            <p className="text-sm text-muted-foreground mt-0.5 truncate">{t.sandbox.subtitle}</p>
           </div>
         </div>
-        <div className="flex flex-wrap items-center gap-2 sm:gap-3">
+        <div className="flex items-center gap-2 sm:gap-3 flex-shrink-0">
           <ModelSelector value={model} onChange={setModel} className="w-40 sm:w-48" />
           {!isLoggedIn && (
             <Link href="/auth"