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/DictionaryWordSelectActivity.h

@@ -0,0 +1,80 @@
+#pragma once
+#include <Epub/Page.h>
+#include <freertos/FreeRTOS.h>
+#include <freertos/semphr.h>
+#include <freertos/task.h>
+
+#include <functional>
+#include <memory>
+#include <string>
+#include <vector>
+
+#include "../Activity.h"
+
+class DictionaryWordSelectActivity final : public Activity {
+ public:
+  explicit DictionaryWordSelectActivity(GfxRenderer& renderer, MappedInputManager& mappedInput,
+                                        std::unique_ptr<Page> page, int fontId, int marginLeft, int marginTop,
+                                        const std::string& cachePath, uint8_t orientation,
+                                        const std::function<void()>& onBack,
+                                        const std::function<void(const std::string&, const std::string&)>& onLookup)
+      : Activity("DictionaryWordSelect", renderer, mappedInput),
+        page(std::move(page)),
+        fontId(fontId),
+        marginLeft(marginLeft),
+        marginTop(marginTop),
+        cachePath(cachePath),
+        orientation(orientation),
+        onBack(onBack),
+        onLookup(onLookup) {}
+
+  void onEnter() override;
+  void onExit() override;
+  void loop() override;
+
+ private:
+  struct WordInfo {
+    std::string text;
+    std::string lookupText;
+    int16_t screenX;
+    int16_t screenY;
+    int16_t width;
+    int16_t row;
+    int continuationIndex;
+    int continuationOf;
+    WordInfo(const std::string& t, int16_t x, int16_t y, int16_t w, int16_t r)
+        : text(t), lookupText(t), screenX(x), screenY(y), width(w), row(r), continuationIndex(-1), continuationOf(-1) {}
+  };
+
+  struct Row {
+    int16_t yPos;
+    std::vector<int> wordIndices;
+  };
+
+  std::unique_ptr<Page> page;
+  int fontId;
+  int marginLeft;
+  int marginTop;
+  std::string cachePath;
+  uint8_t orientation;
+  const std::function<void()> onBack;
+  const std::function<void(const std::string&, const std::string&)> onLookup;
+
+  std::vector<WordInfo> words;
+  std::vector<Row> rows;
+  int currentRow = 0;
+  int currentWordInRow = 0;
+  bool updateRequired = false;
+
+  TaskHandle_t displayTaskHandle = nullptr;
+  SemaphoreHandle_t renderingMutex = nullptr;
+
+  bool isLandscape() const;
+  bool isInverted() const;
+  void extractWords();
+  void mergeHyphenatedWords();
+  void renderScreen();
+  void drawHints();
+  static void taskTrampoline(void* param);
+  [[noreturn]] void displayTaskLoop();
+};