Let users download shared notes as Markdown

The shared notes had no way out: everything lived in the SQLite row and the
only reader was the panel itself. Add two export endpoints and buttons for
them at the bottom of the note panel.

GET /api/notes/{page_id}/export returns one page as .md, and
GET /api/notes/export returns every page as a zip laid out as
<section>/<NN>_<title>.md. Pages are not concatenated into a single file on
purpose - the headings inside a page would collide with whatever heading
level we used as a page separator, so each page stays a verbatim file and the
numeric prefix carries the tab order instead.

Titles reach the filesystem here, so strip the Windows-forbidden characters
and control characters, cap the length, and send Content-Disposition with an
RFC 5987 filename* so Japanese titles survive. The front end deliberately
omits the anchor's download attribute to let that name win.

notes store: flush() now returns a promise so the download can wait for
pending edits to be saved before asking the server for the file.

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

frontend/src/stores/notes.ts

@@ -5,6 +5,8 @@ import {
   createNotePage,
   updateNotePage,
   deleteNotePage,
+  exportNotePage,
+  exportAllNotePages,
   renameNoteSection,
   reorderNotePages,
   type NotePage,
@@ -140,13 +142,28 @@ export const useNotesStore = defineStore('notes', () => {
     }
   }
 
-  /** 保留中の保存を直ちに実行する（タブ切替・パネルを閉じる時など）。 */
-  function flush(): void {
+  /**
+   * 保留中の保存を直ちに実行する（タブ切替・パネルを閉じる時など）。
+   * await すれば保存の完了まで待てる（save() は自分でエラーを握るので reject はしない）。
+   */
+  function flush(): Promise<void> {
     if (saveTimer !== null) {
       clearTimeout(saveTimer)
       saveTimer = null
     }
-    for (const id of [...dirtyIds.value]) void save(id)
+    return Promise.all([...dirtyIds.value].map((id) => save(id))).then(() => undefined)
+  }
+
+  /** 編集中の内容を保存してから、そのページを Markdown としてダウンロードする。 */
+  async function downloadPage(id: number): Promise<void> {
+    await flush()
+    exportNotePage(id)
+  }
+
+  /** 編集中の内容を保存してから、全ページを ZIP としてダウンロードする。 */
+  async function downloadAll(): Promise<void> {
+    await flush()
+    exportAllNotePages()
   }
 
   return {
@@ -164,5 +181,7 @@ export const useNotesStore = defineStore('notes', () => {
     renameSection,
     reorder,
     flush,
+    downloadPage,
+    downloadAll,
   }
 })