feat(opencode): add OpenCode button to AppSwitcher

Add the fourth tab button for OpenCode in the app switcher component,
making the OpenCode providers page accessible from the main navigation.

src/components/AppSwitcher.tsx

@@ -92,6 +92,28 @@ export function AppSwitcher({ activeApp, onSwitch }: AppSwitcherProps) {
         />
         <span>{appDisplayName.gemini}</span>
       </button>
+
+      <button
+        type="button"
+        onClick={() => handleSwitch("opencode")}
+        className={`group inline-flex items-center gap-2 px-3 h-8 rounded-md text-sm font-medium transition-all duration-200 ${
+          activeApp === "opencode"
+            ? "bg-background text-foreground shadow-sm"
+            : "text-muted-foreground hover:text-foreground hover:bg-background/50"
+        }`}
+      >
+        <ProviderIcon
+          icon={appIconName.opencode}
+          name={appDisplayName.opencode}
+          size={iconSize}
+          className={
+            activeApp === "opencode"
+              ? "text-foreground"
+              : "text-muted-foreground group-hover:text-foreground transition-colors"
+          }
+        />
+        <span>{appDisplayName.opencode}</span>
+      </button>
     </div>
   );
 }