feat: Add per-book letterbox fill override

Introduce BookSettings utility for per-book settings stored in
the book's cache directory (book_settings.bin). Add "Letterbox Fill"
option to the EPUB reader menu that cycles Default/Dithered/Solid/None.
At sleep time, the per-book override is loaded and takes precedence
over the global setting for all book types (EPUB, XTC, TXT).

Co-authored-by: Cursor <cursoragent@cursor.com>

src/util/BookSettings.cpp

@@ -0,0 +1,60 @@
+#include "BookSettings.h"
+
+#include <HalStorage.h>
+#include <HardwareSerial.h>
+#include <Serialization.h>
+
+namespace {
+constexpr uint8_t BOOK_SETTINGS_VERSION = 1;
+constexpr uint8_t BOOK_SETTINGS_COUNT = 1;  // Number of persisted fields
+}  // namespace
+
+std::string BookSettings::filePath(const std::string& cachePath) { return cachePath + "/book_settings.bin"; }
+
+BookSettings BookSettings::load(const std::string& cachePath) {
+  BookSettings settings;
+  FsFile f;
+  if (!Storage.openFileForRead("BST", filePath(cachePath), f)) {
+    return settings;
+  }
+
+  uint8_t version;
+  serialization::readPod(f, version);
+  if (version != BOOK_SETTINGS_VERSION) {
+    f.close();
+    return settings;
+  }
+
+  uint8_t fieldCount;
+  serialization::readPod(f, fieldCount);
+
+  // Read fields that exist (supports older files with fewer fields)
+  uint8_t fieldsRead = 0;
+  do {
+    serialization::readPod(f, settings.letterboxFillOverride);
+    if (++fieldsRead >= fieldCount) break;
+    // New fields added here for forward compatibility
+  } while (false);
+
+  f.close();
+  Serial.printf("[%lu] [BST] Loaded book settings from %s (letterboxFill=%d)\n", millis(), filePath(cachePath).c_str(),
+                settings.letterboxFillOverride);
+  return settings;
+}
+
+bool BookSettings::save(const std::string& cachePath, const BookSettings& settings) {
+  FsFile f;
+  if (!Storage.openFileForWrite("BST", filePath(cachePath), f)) {
+    Serial.printf("[%lu] [BST] Could not save book settings!\n", millis());
+    return false;
+  }
+
+  serialization::writePod(f, BOOK_SETTINGS_VERSION);
+  serialization::writePod(f, BOOK_SETTINGS_COUNT);
+  serialization::writePod(f, settings.letterboxFillOverride);
+  // New fields added here
+  f.close();
+
+  Serial.printf("[%lu] [BST] Saved book settings to %s\n", millis(), filePath(cachePath).c_str());
+  return true;
+}