mod: Phase 1 - bring forward mod-exclusive files with ActivityManager migration

Brings ~55 mod-exclusive files to the upstream-based mod/master-resync branch:

Activities (migrated to new ActivityManager pattern):
- Clock/Time: SetTimeActivity, SetTimezoneOffsetActivity, NtpSyncActivity
- Dictionary: DictionaryDefinitionActivity, DictionarySuggestionsActivity,
  DictionaryWordSelectActivity, LookedUpWordsActivity
- Bookmark: EpubReaderBookmarkSelectionActivity
- Book management: BookManageMenuActivity, EndOfBookMenuActivity
- OPDS: OpdsServerListActivity, OpdsSettingsActivity
- Utility: DirectoryPickerActivity, NumericStepperActivity

Utilities (unchanged):
- BookManager, BookSettings, BookmarkStore, BootNtpSync
- Dictionary, LookupHistory, TimeSync, OpdsServerStore

Libraries: PlaceholderCover, TableData, ChapterXPathIndexer
Scripts: inject_mod_version, generate_book_icon, preview_placeholder_cover
Docs: KOReader sync XPath mapping

Migration changes:
- ActivityWithSubactivity -> Activity base class
- Callback constructors -> finish()/setResult() pattern
- enterNewActivity() -> startActivityForResult()
- Activity::RenderLock&& -> RenderLock&&

These files won't compile yet - they reference mod settings and I18n
strings that will be added in subsequent phases.

Made-with: Cursor

src/util/BookSettings.h

@@ -0,0 +1,31 @@
+#pragma once
+#include <cstdint>
+#include <string>
+
+#include "CrossPointSettings.h"
+
+// Per-book settings stored in the book's cache directory.
+// Fields default to sentinel values (0xFF) meaning "use global setting".
+class BookSettings {
+ public:
+  // 0xFF = use global default; otherwise one of SLEEP_SCREEN_LETTERBOX_FILL values (0-2).
+  uint8_t letterboxFillOverride = USE_GLOBAL;
+
+  static constexpr uint8_t USE_GLOBAL = 0xFF;
+
+  // Returns the effective letterbox fill mode: the per-book override if set,
+  // otherwise the global setting from CrossPointSettings.
+  uint8_t getEffectiveLetterboxFill() const {
+    if (letterboxFillOverride != USE_GLOBAL &&
+        letterboxFillOverride < CrossPointSettings::SLEEP_SCREEN_LETTERBOX_FILL_COUNT) {
+      return letterboxFillOverride;
+    }
+    return SETTINGS.sleepScreenLetterboxFill;
+  }
+
+  static BookSettings load(const std::string& cachePath);
+  static bool save(const std::string& cachePath, const BookSettings& settings);
+
+ private:
+  static std::string filePath(const std::string& cachePath);
+};