fix: add formatReleaseNotes helper for markdown changelog rendering (#1468)

- Add formatReleaseNotes() helper to convert releaseNotes to markdown
- Handle string, ReleaseNoteInfo[], null, and undefined formats
- Convert array format to markdown with version headers (## version)
- Use helper in update-available, update-downloaded, and checkForUpdates
- Properly format changelogs when releaseNotes is an array format

Fixes #144

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

apps/frontend/src/main/app-updater.ts

@@ -62,8 +62,13 @@ function formatReleaseNotes(releaseNotes: UpdateInfo['releaseNotes']): string |
   // It's an array of ReleaseNoteInfo objects
   // Format: [{ version: "1.0.0", note: "changes..." }, ...]
   if (Array.isArray(releaseNotes)) {
+    // Return undefined for empty arrays for consistency with null/undefined handling
+    if (releaseNotes.length === 0) {
+      return undefined;
+    }
+
     const formattedNotes = releaseNotes
-      .filter(item => item.note) // Only include entries with notes
+      .filter(item => item.note) // Filter out entries with null/undefined notes
       .map(item => {
         // Each item has version and note properties
         const versionHeader = item.version ? `## ${item.version}\n` : '';