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/stores/toast.ts

@@ -4,7 +4,7 @@ import { defineStore } from "pinia";
 export interface Toast {
   id: number;
   message: string;
-  type: "success" | "error";
+  type: "success" | "error" | "warning";
 }
 
 let nextId = 0;
@@ -12,13 +12,17 @@ let nextId = 0;
 export const useToastStore = defineStore("toast", () => {
   const toasts = ref<Toast[]>([]);
 
-  function show(message: string, type: "success" | "error" = "success") {
+  function show(message: string, type: "success" | "error" | "warning" = "success") {
     const id = nextId++;
     toasts.value.push({ id, message, type });
     setTimeout(() => {
       toasts.value = toasts.value.filter((t) => t.id !== id);
-    }, 3000);
+    }, 4000);
   }
 
-  return { toasts, show };
+  function dismiss(id: number) {
+    toasts.value = toasts.value.filter((t) => t.id !== id);
+  }
+
+  return { toasts, show, dismiss };
 });