Real-time collaboration (Yjs + WebSocket)

- Go WebSocket hub: rooms per document, broadcast updates, persist state
- Yjs integration: connect/disconnect, sync document state
- Collab toggle button in toolbar (Solo/Live)
- When Live: edits broadcast to all connected users in real-time
- Yjs state persisted to SQLite (survives server restart)
- gorilla/websocket dependency added

frontend/src/App.vue

@@ -59,6 +59,9 @@
         <div class="toolbar-right">
           <span class="file-name">{{ currentFile || 'No file open' }}</span>
           <span class="file-meta" v-if="fileMeta">{{ fileMeta }}</span>
+          <button v-if="currentFile" class="toolbar-btn" @click="toggleCollab" :title="collabActive ? 'Disconnect collab' : 'Enable collab'">
+            {{ collabActive ? '👥 Live' : '👤 Solo' }}
+          </button>
           <button v-if="currentFile" class="toolbar-btn" @click="showHistory = !showHistory" title="History">📜</button>
           <button v-if="currentFile" class="toolbar-btn" @click="showShareDialog = !showShareDialog" title="Share">🤝</button>
           <button v-if="currentFile" class="toolbar-btn" @click="aiVerify" title="Verify with AI">🤖 Verify</button>
@@ -248,6 +251,7 @@ import MilkdownEditor from './components/MilkdownEditor.vue'
 import { api, setToken } from './lib/api.js'
 import { renderMarkdown } from './lib/markdown.js'
 import { cacheFile, getCachedFile, addPendingChange, getPendingChanges, clearAllPending } from './lib/offline.js'
+import { connectCollab, disconnectCollab, setCollabContent } from './lib/collab.js'
 
 const authenticated = ref(false)
 const email = ref('')
@@ -277,6 +281,7 @@ const shareMsg = ref('')
 const gitDirty = ref(0)
 const aiResult = ref('')
 const trashItems = ref([])
+const collabActive = ref(false)
 
 // Preferences
 const prefs = ref({ timezone: Intl.DateTimeFormat().resolvedOptions().timeZone, defaultMode: 'split', theme: 'dark' })
@@ -678,6 +683,23 @@ async function aiVerify() {
   }
 }
 
+// ─── Collab ──────────────────────────────────────────────────────────────────
+
+function toggleCollab() {
+  if (collabActive.value) {
+    disconnectCollab()
+    collabActive.value = false
+  } else {
+    if (!currentFile.value) return
+    const fileId = currentFile.value.replace(/[^a-zA-Z0-9]/g, '-')
+    connectCollab(fileId, (newContent) => {
+      content.value = newContent
+    })
+    setCollabContent(content.value)
+    collabActive.value = true
+  }
+}
+
 // ─── Formatting ──────────────────────────────────────────────────────────────
 
 function insertFormat(before, after) {