feat: add font family deletion functionality (#1919)

## Summary

* **What is the goal of this PR?**

1. Adds font family deletion support to the font download activity so
users can remove installed font families directly from the download
list.
2. Address an issue where a font family manually deleted in the file
browser by user will show as "update" in the `FontDownloadActivity`
 

* **What changes are included?**
* Updated button hint label to show `Delete` when deletion is available.
  * Added confirmation before deleting a selected installed font family.
* Refreshed font registry in `fetchAndParseManifest` (for the second
goal above)

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### AI Usage

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src/activities/settings/FontDownloadActivity.h

@@ -86,6 +86,9 @@ class FontDownloadActivity : public Activity {
   void downloadAll();
   static bool computeFileCrc32(const char* path, uint32_t& outCrc);
   bool isDownloadAllSelected() const { return selectedIndex_ == 0 && !families_.empty(); }
+  bool isSelectedFamilyDeletable() const;
+  void promptDeleteSelectedFamily();
+  void onDeleteConfirmationResult(const ActivityResult& result);
   int familyIndexFromList(int listIndex) const { return listIndex - 1; }
   int listItemCount() const { return families_.empty() ? 0 : static_cast<int>(families_.size()) + 1; }
   size_t totalUninstalledSize() const;