Add drag-and-drop reordering for note tabs, and a user-facing guide

Tabs can now be dragged to reorder within a group and to move between
sections in one gesture: dropping on a tab's left/right half inserts
before/after it and adopts that tab's section, while a dedicated zone
past the last group ungroups and appends at the end. A blue indicator
line shows the drop position while dragging.

core/notes.reorder() takes the full new page-id order computed
client-side (NotePanel.vue has the reactive list already) rather than
recomputing positions server-side, validates it against the current
page-id set, sets the moved page's section, then renumbers position.
POST /api/notes/reorder exposes it.

Also adds docs/notes-guide.md, an end-user walkthrough of the shared
notes panel (pages, sections, drag-and-drop, AI interaction) — the
existing docs were all developer-facing specs with no operating
instructions for the feature itself.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/src/stores/notes.ts

@@ -6,6 +6,7 @@ import {
   updateNotePage,
   deleteNotePage,
   renameNoteSection,
+  reorderNotePages,
   type NotePage,
 } from '../api/client'
 
@@ -81,6 +82,21 @@ export const useNotesStore = defineStore('notes', () => {
     }
   }
 
+  /**
+   * タブのドラッグ&ドロップ用。order には全ページIDをドラッグ後の新しい順序で渡す。
+   * section にドロップ先タブの所属セクションを渡せば、並べ替えと同時にセクション間の移動も反映される。
+   */
+  async function reorder(pageId: number, section: string | null, order: number[]): Promise<void> {
+    error.value = null
+    flush()
+    try {
+      await reorderNotePages(pageId, section, order)
+      await load()
+    } catch (e) {
+      error.value = e instanceof Error ? e.message : '並べ替えに失敗しました'
+    }
+  }
+
   async function removePage(id: number): Promise<void> {
     error.value = null
     try {
@@ -146,6 +162,7 @@ export const useNotesStore = defineStore('notes', () => {
     edit,
     setSection,
     renameSection,
+    reorder,
     flush,
   }
 })