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/main.ts

@@ -318,6 +318,7 @@ function createApplicationMenu(): Menu {
         buildSharedMenuItem('reindexText'),
         { type: 'separator' },
         buildSharedMenuItem('metadataDiff'),
+        buildSharedMenuItem('generateSitemap'),
       ],
     },
     {