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>

src/JsonSettingsIO.cpp

@@ -140,6 +140,16 @@ bool JsonSettingsIO::saveSettings(const CrossPointSettings& s, const char* path)
   doc["frontButtonConfirm"] = s.frontButtonConfirm;
   doc["frontButtonLeft"] = s.frontButtonLeft;
   doc["frontButtonRight"] = s.frontButtonRight;
+  // Font family — uses dynamic getter/setter in SettingsList so the generic loop skips it.
+  doc["fontFamily"] = s.fontFamily;
+  // SD card font family name — not in SettingsList, save manually
+  if (s.sdFontFamilyName[0] != '\0') {
+    doc["sdFontFamilyName"] = s.sdFontFamilyName;
+  }
+
+  // Language -- managed by LanguageSelectActivity, not in SettingsList.
+  // Stored as ISO code string ("EN", "DE", ...) for stability across enum reorders.
+  doc["language"] = (s.language < getLanguageCount()) ? LANGUAGE_CODES[s.language] : "EN";
 
   // Language -- managed by LanguageSelectActivity, not in SettingsList.
   // Stored as ISO code string ("EN", "DE", ...) for stability across enum reorders.
@@ -224,6 +234,13 @@ bool JsonSettingsIO::loadSettings(CrossPointSettings& s, const char* json, bool*
       clamp(doc["frontButtonRight"] | (uint8_t)S::FRONT_HW_RIGHT, S::FRONT_BUTTON_HARDWARE_COUNT, S::FRONT_HW_RIGHT);
   CrossPointSettings::validateFrontButtonMapping(s);
 
+  // Font family — uses dynamic getter/setter in SettingsList so the generic loop skips it.
+  s.fontFamily = clamp(doc["fontFamily"] | (uint8_t)0, CrossPointSettings::BUILTIN_FONT_COUNT, 0);
+  // SD card font family name — not in SettingsList, load manually
+  const char* sfn = doc["sdFontFamilyName"] | "";
+  strncpy(s.sdFontFamilyName, sfn, sizeof(s.sdFontFamilyName) - 1);
+  s.sdFontFamilyName[sizeof(s.sdFontFamilyName) - 1] = '\0';
+
   // Language -- stored as code string for stability across enum reorders.
   if (doc["language"].is<const char*>()) {
     s.language = static_cast<uint8_t>(I18n::languageFromCode(doc["language"].as<const char*>()));