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/EpdFont/EpdFontData.h

@@ -129,4 +129,16 @@ typedef struct {
   uint8_t kernRightClassCount;           ///< Number of distinct right classes (matrix cols)
   const EpdLigaturePair* ligaturePairs;  ///< Sorted ligature pair table (nullptr if none)
   uint32_t ligaturePairCount;            ///< Number of entries in ligaturePairs
+
+  /// On-demand glyph loading for fonts that don't keep all glyphs in RAM (e.g. SD card fonts).
+  /// Called by getGlyph() when a codepoint is not found in the interval table.
+  /// Returns a valid EpdGlyph* with correct metadata, or nullptr to fall back to the
+  /// replacement glyph.  The returned pointer is valid until the next glyphMissHandler
+  /// call that causes a ring-buffer eviction — callers must consume it (measure or draw)
+  /// before requesting another missed glyph.
+  const EpdGlyph* (*glyphMissHandler)(void* ctx, uint32_t codepoint);
+
+  /// Context pointer for glyphMissHandler (typically SdCardFont*).  Also used by
+  /// GfxRenderer::getGlyphBitmap() to retrieve overflow bitmaps via SdCardFont.
+  void* glyphMissCtx;
 } EpdFontData;