feat: Add dictionary word lookup feature with cached index

Implements StarDict-based dictionary lookup from the reader menu,
adapted from upstream PR #857 with /.dictionary/ folder path,
std::vector compatibility (PR #802), HTML definition rendering,
orientation-aware button hints, side button hints with CCW text
rotation, sparse index caching to SD card, pronunciation line
filtering, and reorganized reader menu with bookmark stubs.

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

src/activities/reader/LookedUpWordsActivity.h

@@ -0,0 +1,48 @@
+#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 LookedUpWordsActivity final : public ActivityWithSubactivity {
+ public:
+  explicit LookedUpWordsActivity(GfxRenderer& renderer, MappedInputManager& mappedInput, const std::string& cachePath,
+                                 const std::function<void()>& onBack,
+                                 const std::function<void(const std::string&)>& onSelectWord)
+      : ActivityWithSubactivity("LookedUpWords", renderer, mappedInput),
+        cachePath(cachePath),
+        onBack(onBack),
+        onSelectWord(onSelectWord) {}
+
+  void onEnter() override;
+  void onExit() override;
+  void loop() override;
+
+ private:
+  std::string cachePath;
+  const std::function<void()> onBack;
+  const std::function<void(const std::string&)> onSelectWord;
+
+  std::vector<std::string> words;
+  int selectedIndex = 0;
+  bool updateRequired = false;
+  ButtonNavigator buttonNavigator;
+
+  // Delete confirmation state
+  bool deleteConfirmMode = false;
+  bool ignoreNextConfirmRelease = false;
+  int pendingDeleteIndex = 0;
+
+  TaskHandle_t displayTaskHandle = nullptr;
+  SemaphoreHandle_t renderingMutex = nullptr;
+
+  void renderScreen();
+  static void taskTrampoline(void* param);
+  [[noreturn]] void displayTaskLoop();
+};