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/settings.ts

@@ -1,6 +1,7 @@
 import { ref, computed } from "vue";
 import { defineStore } from "pinia";
 import { apiGet, apiPut, apiPost, apiStreamPost } from "@/api/client";
+import { useToastStore } from "@/stores/toast";
 import type { AppSettings } from "@/types/settings";
 
 export const useSettingsStore = defineStore("settings", () => {
@@ -31,6 +32,9 @@ export const useSettingsStore = defineStore("settings", () => {
     loading.value = true;
     try {
       settings.value = await apiPut<AppSettings>("/api/settings", updates);
+    } catch (e) {
+      useToastStore().show("Failed to save settings", "error");
+      throw e;
     } finally {
       loading.value = false;
     }
@@ -55,8 +59,13 @@ export const useSettingsStore = defineStore("settings", () => {
   }
 
   async function deleteModel(model: string) {
-    await apiPost("/api/chat/models/delete", { model });
-    await fetchInstalledModels();
+    try {
+      await apiPost("/api/chat/models/delete", { model });
+      await fetchInstalledModels();
+    } catch (e) {
+      useToastStore().show("Failed to delete model", "error");
+      throw e;
+    }
   }
 
   return {