feat(frontend): Vue 3 workspace UI with light/dark mode

- Vue 3 + Vite + Pinia + Vue Router, Axios client with JWT interceptor
- Auth views (login/register), dashboard with workspace switcher and search
- Nested PageTree sidebar, TipTap rich-text editor with debounced save
- Live collaboration via Socket.IO (remote updates, presence avatars, typing)
- Comments thread per page, invite-members card, member list
- Theme store (light/dark/system) persisted, CSS variables, ThemeToggle
- Multi-stage Dockerfile (build + nginx) for hosting

frontend/index.html

@@ -0,0 +1,12 @@
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="UTF-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <title>openStack</title>
+  </head>
+  <body>
+    <div id="app"></div>
+    <script type="module" src="/src/main.js"></script>
+  </body>
+</html>