feat(ui): hash router — each page has a shareable URL (#35)

- #/health, #/events, #/reaction-logs, etc.
- Browser back/forward navigation works
- Refresh preserves current page
- No new dependencies (hand-rolled hash router)

Closes #35
小橘 🍊（NEKO Team）

packages/engine/ui/src/App.tsx

@@ -27,12 +27,44 @@ type Page =
   | 'reaction-logs'
   | 'request-logs'
 
+const VALID_PAGES: Set<string> = new Set([
+  'health', 'object-defs', 'objects', 'event-defs', 'events',
+  'projection-defs', 'projections', 'reactions', 'api-keys',
+  'reaction-logs', 'request-logs',
+])
+
+function getPageFromHash(): Page {
+  const hash = window.location.hash.replace(/^#\/?/, '')
+  return VALID_PAGES.has(hash) ? (hash as Page) : 'health'
+}
+
+function setHash(page: Page) {
+  window.history.pushState(null, '', `#/${page}`)
+}
+
 function App() {
-  const [page, setPage] = useState<Page>('health')
+  const [page, setPageState] = useState<Page>(getPageFromHash)
   const [needsAuth, setNeedsAuth] = useState(false)
   const [tokenInput, setTokenInput] = useState('')
   const [checking, setChecking] = useState(true)
 
+  // Sync hash → state on popstate (browser back/forward)
+  useEffect(() => {
+    const onHashChange = () => setPageState(getPageFromHash())
+    window.addEventListener('hashchange', onHashChange)
+    window.addEventListener('popstate', onHashChange)
+    return () => {
+      window.removeEventListener('hashchange', onHashChange)
+      window.removeEventListener('popstate', onHashChange)
+    }
+  }, [])
+
+  // Navigate: update hash + state
+  const setPage = (p: Page) => {
+    setHash(p)
+    setPageState(p)
+  }
+
   useEffect(() => {
     setTimeout(() => {
       if (!getToken()) {