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/components/ToastNotification.vue

@@ -13,7 +13,8 @@ const toastStore = useToastStore();
         class="toast"
         :class="`toast--${toast.type}`"
       >
-        {{ toast.message }}
+        <span class="toast-msg">{{ toast.message }}</span>
+        <button class="toast-close" @click="toastStore.dismiss(toast.id)">&times;</button>
       </div>
     </transition-group>
   </div>
@@ -30,19 +31,47 @@ const toastStore = useToastStore();
   gap: 0.5rem;
 }
 .toast {
-  padding: 0.75rem 1.25rem;
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+  padding: 0.75rem 1rem;
   border-radius: 6px;
   color: #fff;
   font-size: 0.9rem;
   box-shadow: 0 2px 8px var(--color-shadow);
   min-width: 200px;
 }
+.toast-msg {
+  flex: 1;
+}
+.toast-close {
+  background: none;
+  border: none;
+  color: rgba(255, 255, 255, 0.7);
+  cursor: pointer;
+  font-size: 1.2rem;
+  line-height: 1;
+  padding: 0 0.15rem;
+}
+.toast-close:hover {
+  color: #fff;
+}
 .toast--success {
   background: var(--color-toast-success);
 }
 .toast--error {
   background: var(--color-toast-error);
 }
+.toast--warning {
+  background: var(--color-warning);
+  color: #1a1a1a;
+}
+.toast--warning .toast-close {
+  color: rgba(0, 0, 0, 0.5);
+}
+.toast--warning .toast-close:hover {
+  color: #1a1a1a;
+}
 .toast-enter-active,
 .toast-leave-active {
   transition: all 0.3s ease;