feat: add sitemap generator to Blog menu

Add a "Generate Sitemap" function to the Blog menu that generates a
standard XML sitemap in the project's html/ folder. The sitemap includes
entries for all published posts, archive pages (year, month, day),
category pages, and tag pages using the preview server URL structure.
Runs as a background task with progress tracking via the task manager.

https://claude.ai/code/session_01PdJyxeeNGf4Bkxvq86GVaZ

src/main/shared/electronApi.ts

@@ -589,6 +589,16 @@ export interface ElectronAPI {
     syncDbToFile: (postIds: string[], groupLabel: string) => Promise<{ success: number; failed: number }>;
     syncFileToDb: (postIds: string[], field: string, groupLabel: string) => Promise<{ success: number; failed: number }>;
   };
+  blog: {
+    generateSitemap: () => Promise<{
+      path: string;
+      urlCount: number;
+      postCount: number;
+      tagCount: number;
+      categoryCount: number;
+      archiveCount: number;
+    }>;
+  };
   chat: {
     // API Key Management
     checkReady: () => Promise<ChatReadyStatus>;