Add multi-user auth, background generation, and chat UX improvements

Phase 5: Multi-user authentication with session cookies, bcrypt passwords,
first-user-is-admin pattern, per-user data isolation, backup/restore,
Docker Swarm production stack with secrets and network isolation.

Phase 5.1: Chat UX improvements:
- Background generation architecture (GenerationBuffer + asyncio task)
  with SSE fan-out, reconnect support, and periodic DB flushes
- LLM-generated conversation titles (first exchange + every 10th message)
- Stop generation button with cancel_event and partial content preservation
- Relative timestamps in sidebar (5m ago, 3h ago, then dates)
- Empty chat auto-cleanup on navigation away
- Save-as-note uses LLM for title generation, tags notes with "chat"
- Summarize-as-note also tags with "chat"

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/App.vue

@@ -1,16 +1,18 @@
 <script setup lang="ts">
-import { ref, computed, onMounted, onUnmounted } from "vue";
+import { ref, computed, onMounted, onUnmounted, watch } from "vue";
 import { useRoute } from "vue-router";
 import AppHeader from "@/components/AppHeader.vue";
 import ChatPanel from "@/components/ChatPanel.vue";
 import ToastNotification from "@/components/ToastNotification.vue";
 import { useTheme } from "@/composables/useTheme";
+import { useAuthStore } from "@/stores/auth";
 import { useChatStore } from "@/stores/chat";
 import { useSettingsStore } from "@/stores/settings";
 
 useTheme();
 
 const route = useRoute();
+const authStore = useAuthStore();
 const chatStore = useChatStore();
 const settingsStore = useSettingsStore();
 const chatPanelOpen = ref(false);
@@ -29,23 +31,50 @@ function toggleChatPanel() {
   chatPanelOpen.value = !chatPanelOpen.value;
 }
 
-onMounted(() => {
+function startAppServices() {
   chatStore.startStatusPolling();
   settingsStore.fetchSettings();
+}
+
+function stopAppServices() {
+  chatStore.stopStatusPolling();
+}
+
+onMounted(async () => {
+  await authStore.checkAuth();
+  if (authStore.isAuthenticated) {
+    startAppServices();
+  }
 });
 
+watch(
+  () => authStore.isAuthenticated,
+  (authenticated) => {
+    if (authenticated) {
+      startAppServices();
+    } else {
+      stopAppServices();
+    }
+  }
+);
+
 onUnmounted(() => {
-  chatStore.stopStatusPolling();
+  stopAppServices();
 });
 </script>
 
 <template>
-  <AppHeader @toggle-chat-panel="toggleChatPanel" />
-  <router-view />
-  <ChatPanel
-    v-if="chatPanelOpen"
-    :context-note-id="contextNoteId"
-    @close="chatPanelOpen = false"
-  />
+  <template v-if="authStore.isAuthenticated">
+    <AppHeader @toggle-chat-panel="toggleChatPanel" />
+    <router-view />
+    <ChatPanel
+      v-if="chatPanelOpen"
+      :context-note-id="contextNoteId"
+      @close="chatPanelOpen = false"
+    />
+  </template>
+  <template v-else>
+    <router-view />
+  </template>
   <ToastNotification />
 </template>