style: fix all cppcheck warnings

- Fix ignored return value in TxtReaderActivity
- Remove unused variables across multiple files
- Add const correctness to DictionaryMargins and EpubReaderActivity
- Replace inefficient substr patterns with resize+append
- Use STL algorithms (find_if, any_of, copy_if, transform) where applicable
- Remove dead sync placeholder code in EpubReaderChapterSelectionActivity
- Add cppcheck suppression for ValueFlow analysis limitation

Resolves all low, medium, and high severity cppcheck defects.

src/activities/reader/EpubReaderActivity.cpp

@@ -386,9 +386,7 @@ void EpubReaderActivity::loop() {
         [this](QuickMenuAction action) {
           // Cache values before exitActivity
           EpubReaderActivity* self = this;
-          GfxRenderer& cachedRenderer = renderer;
-          MappedInputManager& cachedMappedInput = mappedInput;
-          Section* cachedSection = section.get();
+          const Section* cachedSection = section.get();
           SemaphoreHandle_t cachedMutex = renderingMutex;
           
           exitActivity();
@@ -938,7 +936,8 @@ void EpubReaderActivity::renderEndOfBookPrompt() {
   // Book title (truncated if needed)
   std::string bookTitle = epub->getTitle();
   if (bookTitle.length() > 30) {
-    bookTitle = bookTitle.substr(0, 27) + "...";
+    bookTitle.resize(27);
+    bookTitle += "...";
   }
   renderer.drawCenteredText(UI_10_FONT_ID, 120, bookTitle.c_str());