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/notes.ts

@@ -1,6 +1,7 @@
 import { ref } from "vue";
 import { defineStore } from "pinia";
 import { apiGet, apiPost, apiPut, apiDelete } from "@/api/client";
+import { useToastStore } from "@/stores/toast";
 import type { Note, NoteListResponse } from "@/types/note";
 
 export const useNotesStore = defineStore("notes", () => {
@@ -36,6 +37,9 @@ export const useNotesStore = defineStore("notes", () => {
       );
       notes.value = data.notes;
       total.value = data.total;
+    } catch (e) {
+      useToastStore().show("Failed to load notes", "error");
+      throw e;
     } finally {
       loading.value = false;
     }
@@ -62,6 +66,9 @@ export const useNotesStore = defineStore("notes", () => {
     loading.value = true;
     try {
       currentNote.value = await apiGet<Note>(`/api/notes/${id}`);
+    } catch (e) {
+      useToastStore().show("Failed to load note", "error");
+      throw e;
     } finally {
       loading.value = false;
     }
@@ -71,26 +78,40 @@ export const useNotesStore = defineStore("notes", () => {
     title: string;
     body: string;
   }): Promise<Note> {
-    const note = await apiPost<Note>("/api/notes", data);
-    return note;
+    try {
+      return await apiPost<Note>("/api/notes", data);
+    } catch (e) {
+      useToastStore().show("Failed to create note", "error");
+      throw e;
+    }
   }
 
   async function updateNote(
     id: number,
     data: Partial<Pick<Note, "title" | "body">>
   ): Promise<Note> {
-    const note = await apiPut<Note>(`/api/notes/${id}`, data);
-    if (currentNote.value?.id === id) {
-      currentNote.value = note;
+    try {
+      const note = await apiPut<Note>(`/api/notes/${id}`, data);
+      if (currentNote.value?.id === id) {
+        currentNote.value = note;
+      }
+      return note;
+    } catch (e) {
+      useToastStore().show("Failed to update note", "error");
+      throw e;
     }
-    return note;
   }
 
   async function deleteNote(id: number) {
-    await apiDelete(`/api/notes/${id}`);
-    notes.value = notes.value.filter((n) => n.id !== id);
-    if (currentNote.value?.id === id) {
-      currentNote.value = null;
+    try {
+      await apiDelete(`/api/notes/${id}`);
+      notes.value = notes.value.filter((n) => n.id !== id);
+      if (currentNote.value?.id === id) {
+        currentNote.value = null;
+      }
+    } catch (e) {
+      useToastStore().show("Failed to delete note", "error");
+      throw e;
     }
   }
 
@@ -143,26 +164,35 @@ export const useNotesStore = defineStore("notes", () => {
   }
 
   async function convertToTask(noteId: number): Promise<Note> {
-    const note = await apiPost<Note>(
-      `/api/notes/${noteId}/convert-to-task`,
-      {}
-    );
-    // Update local state — the note is now a task
-    if (currentNote.value?.id === noteId) {
-      currentNote.value = note;
+    try {
+      const note = await apiPost<Note>(
+        `/api/notes/${noteId}/convert-to-task`,
+        {}
+      );
+      if (currentNote.value?.id === noteId) {
+        currentNote.value = note;
+      }
+      return note;
+    } catch (e) {
+      useToastStore().show("Failed to convert to task", "error");
+      throw e;
     }
-    return note;
   }
 
   async function convertToNote(noteId: number): Promise<Note> {
-    const note = await apiPost<Note>(
-      `/api/notes/${noteId}/convert-to-note`,
-      {}
-    );
-    if (currentNote.value?.id === noteId) {
-      currentNote.value = note;
+    try {
+      const note = await apiPost<Note>(
+        `/api/notes/${noteId}/convert-to-note`,
+        {}
+      );
+      if (currentNote.value?.id === noteId) {
+        currentNote.value = note;
+      }
+      return note;
+    } catch (e) {
+      useToastStore().show("Failed to convert to note", "error");
+      throw e;
     }
-    return note;
   }
 
   async function fetchBacklinks(