feat: realign zhihu browser callback runtime

Keep Zhihu browser-attached execution on the callback-host path so direct routes, runtime wiring, and service startup stay aligned for the current websocket browser flow.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/browser/backend.rs

@@ -17,6 +17,10 @@ pub trait BrowserBackend: Send + Sync {
     fn supports_eval(&self) -> bool {
         true
     }
+
+    fn supports_live_input(&self) -> bool {
+        false
+    }
 }
 
 impl<T: BrowserBackend + ?Sized> BrowserBackend for Arc<T> {
@@ -36,4 +40,8 @@ impl<T: BrowserBackend + ?Sized> BrowserBackend for Arc<T> {
     fn supports_eval(&self) -> bool {
         self.as_ref().supports_eval()
     }
+
+    fn supports_live_input(&self) -> bool {
+        self.as_ref().supports_live_input()
+    }
 }