feat(workspace): stack hashtag back button above heading and add dedicated hashtag markdown note (#tag.md)

frontend/src/lib/api.ts

@@ -442,6 +442,20 @@ export const api = {
     };
   },
 
+  async updateTagPageNote(
+    tag: string,
+    content: string
+  ): Promise<{ tag: string; content: string; updatedAt: string }> {
+    const clean = encodeURIComponent(tag.replace(/^#/, ''));
+    return request<{ tag: string; content: string; updatedAt: string }>(
+      `/api/v1/workspace/tags/${clean}/note`,
+      {
+        method: 'PUT',
+        body: JSON.stringify({ content }),
+      }
+    );
+  },
+
   // --- WORKSPACE DOCUMENTS ---
   async getDocuments(): Promise<Document[]> {
     const res = await request<Document[]>('/api/v1/documents');