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/preload.ts

@@ -249,6 +249,11 @@ export const electronAPI: ElectronAPI = {
     syncFileToDb: (postIds: string[], field: string, groupLabel: string) => ipcRenderer.invoke('metadataDiff:syncFileToDb', postIds, field, groupLabel),
   },
 
+  // Blog operations
+  blog: {
+    generateSitemap: () => ipcRenderer.invoke('blog:generateSitemap'),
+  },
+
   // AI Chat (OpenCode Zen API integration)
   chat: {
     // API Key Management