feat: Add custom font selection from SD card

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

lib/EpdFont/SdFontFormat.h

@@ -0,0 +1,79 @@
+/**
+ * .epdfont Binary Font Format Specification
+ *
+ * This format is designed for on-demand loading from SD card
+ * with minimal RAM usage on embedded devices.
+ *
+ * File Layout:
+ * ┌─────────────────────────────────────────────────────┐
+ * │ Header (32 bytes)                                   │
+ * ├─────────────────────────────────────────────────────┤
+ * │ Intervals[] (intervalCount × 12 bytes)              │
+ * ├─────────────────────────────────────────────────────┤
+ * │ Glyphs[] (glyphCount × 16 bytes)                    │
+ * ├─────────────────────────────────────────────────────┤
+ * │ Bitmap data (variable size)                         │
+ * └─────────────────────────────────────────────────────┘
+ */
+
+#pragma once
+#include <cstdint>
+
+// Magic number: "EPDF" in little-endian
+#define EPDFONT_MAGIC 0x46445045
+
+// Current format version
+#define EPDFONT_VERSION 1
+
+#pragma pack(push, 1)
+
+/**
+ * File header - 32 bytes
+ */
+struct EpdFontHeader {
+  uint32_t magic;            // 0x46445045 ("EPDF")
+  uint16_t version;          // Format version (1)
+  uint8_t is2Bit;            // 1 = 2-bit grayscale, 0 = 1-bit
+  uint8_t reserved1;         // Reserved for alignment
+  uint8_t advanceY;          // Line height
+  int8_t ascender;           // Max height above baseline
+  int8_t descender;          // Max depth below baseline (negative)
+  uint8_t reserved2;         // Reserved for alignment
+  uint32_t intervalCount;    // Number of unicode intervals
+  uint32_t glyphCount;       // Total number of glyphs
+  uint32_t intervalsOffset;  // File offset to intervals array
+  uint32_t glyphsOffset;     // File offset to glyphs array
+  uint32_t bitmapOffset;     // File offset to bitmap data
+};
+
+/**
+ * Unicode interval - 12 bytes
+ * Same as EpdUnicodeInterval but with explicit packing
+ */
+struct EpdFontInterval {
+  uint32_t first;   // First unicode code point
+  uint32_t last;    // Last unicode code point
+  uint32_t offset;  // Index into glyph array
+};
+
+/**
+ * Glyph data - 16 bytes
+ * Same as EpdGlyph but with explicit packing
+ */
+struct EpdFontGlyph {
+  uint8_t width;        // Bitmap width in pixels
+  uint8_t height;       // Bitmap height in pixels
+  uint8_t advanceX;     // Horizontal advance
+  uint8_t reserved;     // Reserved for alignment
+  int16_t left;         // X offset from cursor
+  int16_t top;          // Y offset from cursor
+  uint32_t dataLength;  // Bitmap data size in bytes
+  uint32_t dataOffset;  // Offset into bitmap section
+};
+
+#pragma pack(pop)
+
+// Sanity checks for struct sizes
+static_assert(sizeof(EpdFontHeader) == 32, "EpdFontHeader must be 32 bytes");
+static_assert(sizeof(EpdFontInterval) == 12, "EpdFontInterval must be 12 bytes");
+static_assert(sizeof(EpdFontGlyph) == 16, "EpdFontGlyph must be 16 bytes");