Allow users to select custom fonts (.epdfont files) from the
/.crosspoint/fonts/ directory on the SD card for EPUB/TXT reading.
Features:
- New FontSelectionActivity for browsing and selecting fonts
- SdFont and SdFontFamily classes for loading fonts from SD card
- Dynamic font reloading without device reboot
- Reader cache invalidation when font changes
- Hash-based font ID generation for proper cache management
The custom fonts use the .epdfont binary format which supports:
- 2-bit antialiasing for smooth text rendering
- Efficient on-demand glyph loading with LRU cache
- Memory-optimized design for ESP32-C3 constraints
## Summary
* Update EpdFontFamily::Style to be u8 instead of u32 (saving 3 bytes
per word)
* Update layout width/height to be u16 from int
* Update page element count to be u16 from u32
* Update text block element count to be u16 from u32
* Bumped section bin version to version 8
