style(app): update header and app switcher styling

App.tsx:
- Replace glass-header with explicit bg-background/80 backdrop-blur
- Update navigation button container to use bg-muted

AppSwitcher:
- Replace hardcoded gray colors with semantic muted/foreground tokens
- Ensure consistent dark mode support via CSS variables
- Add group class for better hover state transitions

src/components/AppSwitcher.tsx

@@ -24,14 +24,14 @@ export function AppSwitcher({ activeApp, onSwitch }: AppSwitcherProps) {
   };
 
   return (
-    <div className="inline-flex bg-gray-100 dark:bg-gray-800 rounded-lg p-1 gap-1">
+    <div className="inline-flex bg-muted rounded-lg p-1 gap-1">
       <button
         type="button"
         onClick={() => handleSwitch("claude")}
         className={`group inline-flex items-center gap-2 px-3 py-2 rounded-md text-sm font-medium transition-all duration-200 ${
           activeApp === "claude"
-            ? "bg-white text-gray-900 shadow-sm dark:bg-gray-900 dark:text-gray-100"
-            : "text-gray-500 hover:text-gray-900 hover:bg-white/50 dark:text-gray-400 dark:hover:text-gray-100 dark:hover:bg-gray-800/60"
+            ? "bg-background text-foreground shadow-sm"
+            : "text-muted-foreground hover:text-foreground hover:bg-background/50"
         }`}
       >
         <ProviderIcon
@@ -41,7 +41,7 @@ export function AppSwitcher({ activeApp, onSwitch }: AppSwitcherProps) {
           className={
             activeApp === "claude"
               ? "text-foreground"
-              : "text-gray-500 dark:text-gray-400 group-hover:text-foreground transition-colors"
+              : "text-muted-foreground group-hover:text-foreground transition-colors"
           }
         />
         <span>{appDisplayName.claude}</span>
@@ -50,10 +50,10 @@ export function AppSwitcher({ activeApp, onSwitch }: AppSwitcherProps) {
       <button
         type="button"
         onClick={() => handleSwitch("codex")}
-        className={`inline-flex items-center gap-2 px-3 py-2 rounded-md text-sm font-medium transition-all duration-200 ${
+        className={`group inline-flex items-center gap-2 px-3 py-2 rounded-md text-sm font-medium transition-all duration-200 ${
           activeApp === "codex"
-            ? "bg-white text-gray-900 shadow-sm dark:bg-gray-900 dark:text-gray-100"
-            : "text-gray-500 hover:text-gray-900 hover:bg-white/50 dark:text-gray-400 dark:hover:text-gray-100 dark:hover:bg-gray-800/60"
+            ? "bg-background text-foreground shadow-sm"
+            : "text-muted-foreground hover:text-foreground hover:bg-background/50"
         }`}
       >
         <ProviderIcon
@@ -63,7 +63,7 @@ export function AppSwitcher({ activeApp, onSwitch }: AppSwitcherProps) {
           className={
             activeApp === "codex"
               ? "text-foreground"
-              : "text-gray-500 dark:text-gray-400 group-hover:text-foreground transition-colors"
+              : "text-muted-foreground group-hover:text-foreground transition-colors"
           }
         />
         <span>{appDisplayName.codex}</span>
@@ -72,10 +72,10 @@ export function AppSwitcher({ activeApp, onSwitch }: AppSwitcherProps) {
       <button
         type="button"
         onClick={() => handleSwitch("gemini")}
-        className={`inline-flex items-center gap-2 px-3 py-2 rounded-md text-sm font-medium transition-all duration-200 ${
+        className={`group inline-flex items-center gap-2 px-3 py-2 rounded-md text-sm font-medium transition-all duration-200 ${
           activeApp === "gemini"
-            ? "bg-white text-gray-900 shadow-sm dark:bg-gray-900 dark:text-gray-100"
-            : "text-gray-500 hover:text-gray-900 hover:bg-white/50 dark:text-gray-400 dark:hover:text-gray-100 dark:hover:bg-gray-800/60"
+            ? "bg-background text-foreground shadow-sm"
+            : "text-muted-foreground hover:text-foreground hover:bg-background/50"
         }`}
       >
         <ProviderIcon
@@ -85,7 +85,7 @@ export function AppSwitcher({ activeApp, onSwitch }: AppSwitcherProps) {
           className={
             activeApp === "gemini"
               ? "text-foreground"
-              : "text-gray-500 dark:text-gray-400 group-hover:text-foreground transition-colors"
+              : "text-muted-foreground group-hover:text-foreground transition-colors"
           }
         />
         <span>{appDisplayName.gemini}</span>