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/Utf8/Utf8.h

@@ -15,6 +15,24 @@ void utf8TruncateChars(std::string& str, size_t numChars);
 // incomplete trailing bytes are excluded.
 int utf8SafeTruncateBuffer(const char* buf, int len);
 
+// Returns true for CJK characters that allow line breaks on either side without hyphenation.
+// Covers CJK Unified Ideographs, Hiragana, Katakana, Hangul Syllables, CJK punctuation,
+// and fullwidth forms — the ranges where word boundaries are implicit per character.
+inline bool utf8IsCjkBreakable(const uint32_t cp) {
+  return (cp >= 0x3000 && cp <= 0x303F)        // CJK Symbols and Punctuation
+         || (cp >= 0x3040 && cp <= 0x309F)     // Hiragana
+         || (cp >= 0x30A0 && cp <= 0x30FF)     // Katakana
+         || (cp >= 0x3400 && cp <= 0x4DBF)     // CJK Extension A
+         || (cp >= 0x4E00 && cp <= 0x9FFF)     // CJK Unified Ideographs
+         || (cp >= 0xAC00 && cp <= 0xD7AF)     // Hangul Syllables
+         || (cp >= 0xF900 && cp <= 0xFAFF)     // CJK Compatibility Ideographs
+         || (cp >= 0xFE30 && cp <= 0xFE4F)     // CJK Compatibility Forms
+         || (cp >= 0xFF01 && cp <= 0xFF60)     // Fullwidth Latin / Punctuation
+         || (cp >= 0xFF65 && cp <= 0xFFEF)     // Halfwidth Katakana / Hangul
+         || (cp >= 0x20000 && cp <= 0x2A6DF)   // CJK Extension B
+         || (cp >= 0x2A700 && cp <= 0x2B73F);  // CJK Extension C
+}
+
 // Returns true for Unicode combining diacritical marks that should not advance the cursor.
 inline bool utf8IsCombiningMark(const uint32_t cp) {
   return (cp >= 0x0300 && cp <= 0x036F)      // Combining Diacritical Marks