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/renderer/App.tsx

@@ -277,6 +277,17 @@ const App: React.FC = () => {
       }) || (() => {})
     );
 
+    unsubscribers.push(
+      window.electronAPI?.on('menu:generateSitemap', async () => {
+        try {
+          await window.electronAPI?.blog.generateSitemap();
+        } catch (error) {
+          console.error('Sitemap generation failed:', error);
+          showToast.error('Sitemap generation failed');
+        }
+      }) || (() => {})
+    );
+
     // Import completion event - refresh posts and media stores
     unsubscribers.push(
       window.electronAPI?.import.onComplete(async (data) => {