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/components/NotePanel.vue

@@ -212,10 +212,26 @@ onUnmounted(() => {
       <button class="create-btn" @click="store.addPage()">＋ 最初のページを作成</button>
     </div>
 
+    <div class="actions">
+      <button
+        class="action-btn"
+        :disabled="store.activeId === null"
+        title="開いているページを Markdown ファイルとして保存"
+        @click="store.activeId !== null && store.downloadPage(store.activeId)"
+      >⬇ このページ (.md)</button>
+      <button
+        class="action-btn"
+        :disabled="store.pages.length === 0"
+        title="全ページを ZIP（1ページ1 Markdown、セクションはフォルダ）として保存"
+        @click="store.downloadAll()"
+      >⬇ すべて (.zip)</button>
+    </div>
+
     <div class="hint">
       ページのタイトル一覧はAIのシステムプロンプトに常に含まれます。本文はAIが必要に応じて読み出します。
       🏷 でページをセクションにまとめられます（セクション名をクリックすると変更・解除）。
       タブはドラッグで並べ替え・他のセクションへの移動ができます。
+      ⬇ でノートを手元にダウンロードできます（未保存の編集は保存してから書き出します）。
     </div>
   </div>
 </template>
@@ -355,6 +371,26 @@ onUnmounted(() => {
   font-size: 12px;
   cursor: pointer;
 }
+.actions {
+  flex-shrink: 0;
+  display: flex;
+  justify-content: flex-end;
+  gap: 6px;
+  padding: 6px 8px;
+  border-top: 1px solid #2a2a2a;
+}
+.action-btn {
+  background: #1f1f1f;
+  color: #aaa;
+  border: 1px solid #333;
+  border-radius: 6px;
+  padding: 4px 10px;
+  font-size: 11px;
+  cursor: pointer;
+  white-space: nowrap;
+}
+.action-btn:hover:not(:disabled) { background: #2a2a2a; color: #93c5fd; border-color: #444; }
+.action-btn:disabled { opacity: 0.4; cursor: not-allowed; }
 .hint {
   flex-shrink: 0;
   padding: 8px 10px;