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

mod/docs/todo.md

@@ -0,0 +1,16 @@
+- [X] Sleep screen tweaks
+- [ ] Better home screen (covers/recents)
+- [ ] Firmware flashing screen
+- [X] Process/render all covers/thumbs when opening book for first time
+- [ ] Companion Android app
+    - [ ] Smart fill letterbox
+- [X] Bookmark skeleton
+- [X] Dictionary skeleton
+- [ ] Quick menu
+    - [X] Bookmarks
+    - [X] Dictionary
+    - [X] Rotate screen
+    - [ ] Archive/Delete book
+
+Optional?
+- [ ] Device bezel offsets