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/Dockerfile

@@ -0,0 +1,17 @@
+# Build stage
+FROM node:20-alpine AS build
+WORKDIR /app
+ARG VITE_API_URL=/api
+ARG VITE_SOCKET_URL=""
+ENV VITE_API_URL=$VITE_API_URL
+ENV VITE_SOCKET_URL=$VITE_SOCKET_URL
+COPY package*.json ./
+RUN npm ci
+COPY . .
+RUN npm run build
+
+# Serve stage
+FROM nginx:alpine
+COPY --from=build /app/dist /usr/share/nginx/html
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+EXPOSE 80