feat: Integrate PR #857 dictionary intelligence and sub-activity refactor

Pull in the full feature update from PR #857 while preserving fork
advantages (HTML parsing, custom drawHints, PageForward/PageBack,
cache management, stardictCmp, /.dictionary/ paths).

- Add morphological stemming (getStemVariants), Levenshtein edit
  distance, and fuzzy matching (findSimilar) to Dictionary
- Create DictionarySuggestionsActivity for "Did you mean?" flow
- Add onDone callback to DictionaryDefinitionActivity for direct
  exit-to-reader via "Done" button
- Refactor DictionaryWordSelectActivity to ActivityWithSubactivity
  with cascading lookup (exact → stems → suggestions → not found),
  en-dash/em-dash splitting, and cross-page hyphenation
- Refactor LookedUpWordsActivity with reverse-chronological order,
  inline cascading lookup, UITheme-aware rendering, and sub-activities
- Simplify EpubReaderActivity LOOKUP/LOOKED_UP_WORDS handlers

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

src/activities/reader/DictionaryDefinitionActivity.cpp

@@ -450,8 +450,16 @@ void DictionaryDefinitionActivity::loop() {
     updateRequired = true;
   }
 
-  if (mappedInput.wasReleased(MappedInputManager::Button::Back) ||
-      mappedInput.wasReleased(MappedInputManager::Button::Confirm)) {
+  if (mappedInput.wasReleased(MappedInputManager::Button::Confirm)) {
+    if (onDone) {
+      onDone();
+    } else {
+      onBack();
+    }
+    return;
+  }
+
+  if (mappedInput.wasReleased(MappedInputManager::Button::Back)) {
     onBack();
     return;
   }
@@ -491,8 +499,8 @@ void DictionaryDefinitionActivity::renderScreen() {
                       renderer.getScreenHeight() - 50, pageInfo.c_str());
   }
 
-  // Button hints (bottom face buttons — hide Confirm stub like Home Screen)
-  const auto labels = mappedInput.mapLabels("\xC2\xAB Back", "", "\xC2\xAB Page", "Page \xC2\xBB");
+  // Button hints (bottom face buttons)
+  const auto labels = mappedInput.mapLabels("\xC2\xAB Back", onDone ? "Done" : "", "\xC2\xAB Page", "Page \xC2\xBB");
   GUI.drawButtonHints(renderer, labels.btn1, labels.btn2, labels.btn3, labels.btn4);
 
   // Side button hints (drawn in portrait coordinates for correct placement)