feat: verify CRC32 checksum for font files (#1904)

## Summary

* **What is the goal of this PR?**  
Add end-to-end integrity verification for downloaded font files by
including CRC32 checksums in the font manifest and validating downloaded
`.cpfont` files on device.

* **What changes are included?**  
- `generate-font-manifest.py`: compute and include `crc32` for each
`.cpfont` asset in the generated `fonts.json` manifest.
- `FontDownloadActivity.h`: extend manifest file metadata with `crc32`
and declare checksum helper.
- `FontDownloadActivity.cpp`: parse `crc32` from manifest, compute CRC32
of downloaded files using `esp_rom_crc32_le`.

---

### AI Usage

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Did you use AI tools to help write this code? _**YES**_

src/activities/settings/FontDownloadActivity.h

@@ -50,6 +50,7 @@ class FontDownloadActivity : public Activity {
   struct ManifestFile {
     std::string name;
     size_t size = 0;
+    uint32_t crc32 = 0;
   };
 
   struct ManifestFamily {
@@ -83,6 +84,7 @@ class FontDownloadActivity : public Activity {
   bool fetchAndParseManifest();
   void downloadFamily(ManifestFamily& family);
   void downloadAll();
+  static bool computeFileCrc32(const char* path, uint32_t& outCrc);
   bool isDownloadAllSelected() const { return selectedIndex_ == 0 && !families_.empty(); }
   int familyIndexFromList(int listIndex) const { return listIndex - 1; }
   int listItemCount() const { return families_.empty() ? 0 : static_cast<int>(families_.size()) + 1; }