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**_

scripts/generate-font-manifest.py

@@ -21,6 +21,7 @@ import json
 import os
 import struct
 import sys
+import zlib
 from pathlib import Path
 
 # Import canonical version constants from the shared file in lib/EpdFont/scripts/
@@ -115,6 +116,15 @@ def parse_filename(filename: str) -> tuple[str, str] | None:
     return family, size_str
 
 
+def compute_crc32(filepath: Path) -> int:
+    """Compute CRC32 of a file, matching esp_rom_crc32_le(0xFFFFFFFF, ...) ^ 0xFFFFFFFF."""
+    crc = 0
+    with open(filepath, "rb") as f:
+        for chunk in iter(lambda: f.read(65536), b""):
+            crc = zlib.crc32(chunk, crc)
+    return crc & 0xFFFFFFFF
+
+
 def scan_cpfont_files(input_dir: Path) -> dict[str, list[Path]]:
     """Scan input directory for .cpfont files, grouped by family name.
 
@@ -164,6 +174,7 @@ def build_manifest(
                 {
                     "name": filepath.name,
                     "size": filepath.stat().st_size,
+                    "crc32": compute_crc32(filepath),
                 }
             )