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/DictionaryDefinitionActivity.h

@@ -0,0 +1,74 @@
+#pragma once
+#include <EpdFontFamily.h>
+#include <freertos/FreeRTOS.h>
+#include <freertos/semphr.h>
+#include <freertos/task.h>
+
+#include <functional>
+#include <string>
+#include <vector>
+
+#include "../Activity.h"
+
+class DictionaryDefinitionActivity final : public Activity {
+ public:
+  explicit DictionaryDefinitionActivity(GfxRenderer& renderer, MappedInputManager& mappedInput,
+                                        const std::string& headword, const std::string& definition, int readerFontId,
+                                        uint8_t orientation, const std::function<void()>& onBack)
+      : Activity("DictionaryDefinition", renderer, mappedInput),
+        headword(headword),
+        definition(definition),
+        readerFontId(readerFontId),
+        orientation(orientation),
+        onBack(onBack) {}
+
+  void onEnter() override;
+  void onExit() override;
+  void loop() override;
+
+ private:
+  // A positioned text segment within a wrapped line (pre-calculated x offset and style).
+  struct Segment {
+    std::string text;
+    int16_t x;
+    EpdFontFamily::Style style;
+  };
+
+  // An intermediate token produced by the HTML parser before word-wrapping.
+  struct TextAtom {
+    std::string text;
+    EpdFontFamily::Style style;
+    bool isNewline;
+    int indent;  // pixels to indent the new line (for nested lists)
+  };
+
+  // Tracks ordered/unordered list nesting during HTML parsing.
+  struct ListState {
+    int counter;   // incremented per <li>, 0 = not yet used
+    bool isAlpha;  // true for list-style-type: lower-alpha
+  };
+
+  std::string headword;
+  std::string definition;
+  int readerFontId;
+  uint8_t orientation;
+  const std::function<void()> onBack;
+
+  std::vector<std::vector<Segment>> wrappedLines;
+  int currentPage = 0;
+  int linesPerPage = 0;
+  int totalPages = 0;
+  bool updateRequired = false;
+  bool firstRender = true;
+
+  TaskHandle_t displayTaskHandle = nullptr;
+  SemaphoreHandle_t renderingMutex = nullptr;
+
+  std::vector<TextAtom> parseHtml(const std::string& html);
+  static std::string decodeEntity(const std::string& entity);
+  static bool isRenderableCodepoint(uint32_t cp);
+  void wrapText();
+  void renderScreen();
+  static void taskTrampoline(void* param);
+  [[noreturn]] void displayTaskLoop();
+};