mod: adapt mod activities to #774 render() pattern

Migrate 5 mod Activity subclasses from old polling-based
display task pattern to the upstream render() super-class
pattern with freeRTOS notification:

- EpubReaderBookmarkSelectionActivity
- DictionaryWordSelectActivity
- DictionarySuggestionsActivity
- DictionaryDefinitionActivity
- LookedUpWordsActivity

Changes: remove own TaskHandle/SemaphoreHandle/updateRequired,
use requestUpdate() + render(RenderLock&&) override, fix
potential deadlocks around enterNewActivity() calls.

Also fix stale conflict marker in EpubReaderMenuActivity.h.

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

src/activities/reader/EpubReaderMenuActivity.h

@@ -82,7 +82,6 @@ class EpubReaderMenuActivity final : public ActivityWithSubactivity {
 
   const std::function<void(uint8_t)> onBack;
   const std::function<void(MenuAction)> onAction;
-<<<<<<< HEAD
 
   // Map the internal override value to an index into letterboxFillLabels.
   int letterboxFillToIndex() const {