Reduce legacy route flicker

Keep React-owned pages out of the legacy page activator during initial bootstrap, and switch the visible React host before paint when the shell mounts.

That removes the refresh flash on /issues while preserving the legacy-page behavior and browser-history stability.

Verified with the router tests and the issues smoke suite.

webui/src/platform/shell/route-controllers.tsx

@@ -1,5 +1,5 @@
 import { useRouter } from '@tanstack/react-router';
-import { useEffect, useState } from 'react';
+import { useEffect, useLayoutEffect, useState } from 'react';
 
 import { getProfileHomePath, getShellBridge, type ShellPageId } from './bridge';
 
@@ -42,6 +42,14 @@ export function useReactPageShell(pageId: ShellPageId) {
   const bridge = useShellBridge();
   const router = useRouter();
 
+  useLayoutEffect(() => {
+    if (!bridge) return;
+    if (!bridge.isPageAllowed(pageId)) return;
+
+    bridge.setActivePageChrome(pageId);
+    bridge.showReactHost(pageId);
+  }, [bridge, pageId]);
+
   useEffect(() => {
     if (!bridge) return;
 
@@ -49,9 +57,6 @@ export function useReactPageShell(pageId: ShellPageId) {
       void router.navigate({ href: getProfileHomePath(bridge), replace: true });
       return;
     }
-
-    bridge.setActivePageChrome(pageId);
-    bridge.showReactHost(pageId);
   }, [bridge, pageId, router]);
 
   return bridge;

webui/static/search.js

@@ -1180,6 +1180,15 @@ async function loadInitialData() {
         // Always apply the target page to the legacy shell chrome.
         // When the router is present, skipRouteChange keeps the URL stable
         // while still syncing the active page/nav state for direct loads.
+        const router = getWebRouter();
+        const route = router?.routeManifest?.find((entry) => entry.pageId === targetPage);
+
+        if (route?.kind === 'react') {
+            showReactHost(targetPage);
+            setActivePageChrome(targetPage);
+            return;
+        }
+
         navigateToPage(targetPage, { skipRouteChange: true, forceReload: true });
     } catch (error) {
         console.error('Error loading initial data:', error);