feat: add SD card font support with on-device download and web management

Add a complete SD card font subsystem that enables users to install and
use custom fonts beyond the three built-in families. This combines the
back-end firmware support (#1327) with the font configuration, build
pipeline, CI distribution, and user-facing management UI (#1392).

Core font system:
- Custom .cpfont binary format (v4) with multi-style support (regular,
  bold, italic, bold-italic) packed into a single file per size
- On-demand glyph loading from SD card with two-pass prewarm rendering
  to bulk-read glyphs per page, achieving near-flash performance for
  Latin text (~697ms vs ~681ms) and viable CJK rendering (~32% slower)
- Persistent advance cache for layout measurement without SD I/O
- Overflow ring buffer for glyph cache misses during rendering
- Memory-conscious design: only advance tables kept in RAM; glyph
  bitmaps, kern tables, and ligatures loaded on demand from SD

Font management:
- On-device WiFi download from GitHub Releases with manifest-based
  discovery, install/update detection, and progress UI
- Web interface font upload, listing, and deletion via /fonts page
- Manual SD card copy to /fonts/ or /.fonts/ directories
- Font selection integrated into Settings > Reader > Font Family

Build pipeline:
- Declarative YAML config (sd-fonts.yaml) as single source of truth
  for the 17-family font library (serif, sans, mono, accessibility)
- Python converter (fontconvert_sdcard.py) for TTF/OTF to .cpfont with
  FreeType rasterization, class-based kerning, and ligature extraction
- Parallel build orchestrator with variable font instance extraction
- CI workflow publishing versioned + stable releases to a dedicated
  crosspoint-fonts repository with auto-incrementing revision tags
- Centralized version constants (cpfont_version.py) shared across
  build tooling and CI, with firmware headers as manual sync points

Additional fixes:
- CJK characters no longer get hyphens inserted at line breaks
- Advance table eliminates 30+ second stalls during CJK section
  indexing for paragraphs with >512 unique codepoints

Closes #930

Co-authored-by: Zach Nelson <zach@zdnelson.com>
Co-authored-by: Justin <itsthisjustin@users.noreply.github.com>
Co-authored-by: jpirnay <jens@pirnay.com>
Co-authored-by: mcrosson <kemonine@kemonine.info>

lib/I18n/translations/english.yaml

@@ -309,6 +309,21 @@ STR_DELETE_CONFIRM: "Delete this server?"
 STR_OPDS_SERVERS: "OPDS Servers"
 STR_AUTO_TURN_ENABLED: "Auto Turn Enabled: "
 STR_AUTO_TURN_PAGES_PER_MIN: "Auto Turn (Pages Per Minute)"
+STR_DOWNLOAD_FONTS: "Download Fonts"
+STR_FONT_DOWNLOAD: "Font Download"
+STR_LOADING_FONT_LIST: "Loading font list..."
+STR_NO_FONTS_AVAILABLE: "No fonts available"
+STR_FONT_INSTALLED: "Font installed!"
+STR_FONT_INSTALL_FAILED: "Font installation failed"
+STR_INSTALLED: "Installed"
+STR_CONFIRM_DOWNLOAD_PROMPT: "Download?"
+STR_SD_CARD_FULL: "Insufficient SD card space"
+STR_FILES_LABEL: "Files: "
+STR_SIZE_LABEL: "Size: "
+STR_REDOWNLOAD: "Re-download"
+STR_DOWNLOAD_ALL: "Download / Update All"
+STR_ALL_FONTS_INSTALLED: "All fonts installed!"
+STR_UPDATE_AVAILABLE: "Update"
 STR_CRASH_TITLE: "System Crash"
 STR_CRASH_DESCRIPTION: "A detailed report was saved to crash_report.txt. Please include this file in your bug report."
 STR_CRASH_REASON: "Crash reason:"