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/assets/theme.css

@@ -110,3 +110,21 @@ button:focus-visible {
   outline: none;
   box-shadow: var(--focus-ring);
 }
+
+/* Responsive breakpoints: 480px (phone), 768px (tablet), 1024px (desktop) */
+@media (max-width: 768px) {
+  .hide-mobile {
+    display: none !important;
+  }
+  button,
+  [role="button"],
+  .btn-new-conv,
+  .btn-send {
+    min-height: 44px;
+  }
+}
+@media (min-width: 769px) {
+  .hide-desktop {
+    display: none !important;
+  }
+}