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/src/App.vue

@@ -0,0 +1,17 @@
+<script setup>
+import { onMounted } from 'vue';
+import { useThemeStore } from './stores/theme';
+import ThemeToggle from './components/ThemeToggle.vue';
+
+const theme = useThemeStore();
+onMounted(() => theme.apply());
+</script>
+
+<template>
+  <div>
+    <div style="position: fixed; top: 12px; right: 12px; z-index: 50">
+      <ThemeToggle />
+    </div>
+    <router-view />
+  </div>
+</template>