feat(ui): center title and balance header layout

Unify title to "CC Switch" to prevent text length jumping during app switching.
Reorganize header as three-column grid with centered title.

src/App.css

@@ -9,13 +9,10 @@
   color: white;
   padding: 0.35rem 2rem 0.45rem;
   display: grid;
-  grid-template-columns: 1fr auto;
+  grid-template-columns: 1fr auto 1fr;
-  grid-template-rows: auto auto;
   grid-template-areas:
-    "title actions"
+    "tabs title actions";
-    "tabs actions";
   align-items: center;
-  row-gap: 0.45rem; /* 间隔增大一点，标题与开关分离 */
   box-shadow: 0 2px 6px rgba(0, 0, 0, 0.15);
   user-select: none;
 }
@@ -85,12 +82,14 @@
   font-weight: 500;
   margin: 0;
   grid-area: title;
+  text-align: center;
 }
 
 .header-actions {
   display: flex;
   gap: 1rem;
   grid-area: actions;
+  justify-self: end;
 }
 
 .refresh-btn,

src/App.tsx

@@ -181,7 +181,7 @@ function App() {
   return (
     <div className="app">
       <header className="app-header">
-        <h1>{activeApp === "claude" ? "Claude Code" : "Codex"} 供应商切换器</h1>
+        <h1>CC Switch</h1>
         <div className="app-tabs">
           <AppSwitcher
             activeApp={activeApp}