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/DictionarySuggestionsActivity.h

@@ -0,0 +1,53 @@
+#pragma once
+#include <freertos/FreeRTOS.h>
+#include <freertos/semphr.h>
+#include <freertos/task.h>
+
+#include <functional>
+#include <string>
+#include <vector>
+
+#include "../ActivityWithSubactivity.h"
+#include "util/ButtonNavigator.h"
+
+class DictionarySuggestionsActivity final : public ActivityWithSubactivity {
+ public:
+  explicit DictionarySuggestionsActivity(GfxRenderer& renderer, MappedInputManager& mappedInput,
+                                         const std::string& originalWord, const std::vector<std::string>& suggestions,
+                                         int readerFontId, uint8_t orientation, const std::string& cachePath,
+                                         const std::function<void()>& onBack, const std::function<void()>& onDone)
+      : ActivityWithSubactivity("DictionarySuggestions", renderer, mappedInput),
+        originalWord(originalWord),
+        suggestions(suggestions),
+        readerFontId(readerFontId),
+        orientation(orientation),
+        cachePath(cachePath),
+        onBack(onBack),
+        onDone(onDone) {}
+
+  void onEnter() override;
+  void onExit() override;
+  void loop() override;
+
+ private:
+  std::string originalWord;
+  std::vector<std::string> suggestions;
+  int readerFontId;
+  uint8_t orientation;
+  std::string cachePath;
+  const std::function<void()> onBack;
+  const std::function<void()> onDone;
+
+  int selectedIndex = 0;
+  bool updateRequired = false;
+  bool pendingBackFromDef = false;
+  bool pendingExitToReader = false;
+  ButtonNavigator buttonNavigator;
+
+  TaskHandle_t displayTaskHandle = nullptr;
+  SemaphoreHandle_t renderingMutex = nullptr;
+
+  void renderScreen();
+  static void taskTrampoline(void* param);
+  [[noreturn]] void displayTaskLoop();
+};