feat: support navigating to primary pages via URL query parameter

Allow users to open a specific primary page by visiting /?page=explore,
/?page=settings, etc.

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

src/PageManager.tsx

@@ -139,6 +139,10 @@ export function PageManager({ maxStackSize = 5 }: { maxStackSize?: number }) {
           navigatePrimaryPage('relay', { url })
         }
       }
+      const page = searchParams.get('page')
+      if (page && page in PRIMARY_PAGE_MAP) {
+        navigatePrimaryPage(page as TPrimaryPageName)
+      }
     }
 
     const onPopState = (e: PopStateEvent) => {