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/boot_sleep/SleepActivity.cpp

@@ -164,8 +164,7 @@ void SleepActivity::renderBitmapSleepScreen(const Bitmap& bitmap, const std::str
   float cropX = 0, cropY = 0;
   int drawWidth = pageWidth;
   int drawHeight = pageHeight;
-  int fillWidth = pageWidth;   // Actual area the image will occupy
-  int fillHeight = pageHeight;
+  int fillWidth, fillHeight;  // Actual area the image will occupy (set per mode)
 
   Serial.printf("[%lu] [SLP] bitmap %d x %d, screen %d x %d\n", millis(), bitmap.getWidth(), bitmap.getHeight(),
                 pageWidth, pageHeight);
@@ -183,9 +182,9 @@ void SleepActivity::renderBitmapSleepScreen(const Bitmap& bitmap, const std::str
     // Don't constrain to screen dimensions - drawBitmap will clip
     drawWidth = 0;
     drawHeight = 0;
-    fillWidth = bitmap.getWidth();
-    fillHeight = bitmap.getHeight();
-    Serial.printf("[%lu] [SLP] ACTUAL mode: centering at %d, %d\n", millis(), x, y);
+    fillWidth = static_cast<int>(bitmap.getWidth());
+    fillHeight = static_cast<int>(bitmap.getHeight());
+    Serial.printf("[%lu] [SLP] ACTUAL mode: centering at %d, %d (fill: %dx%d)\n", millis(), x, y, fillWidth, fillHeight);
   } else if (coverMode == CrossPointSettings::SLEEP_SCREEN_COVER_MODE::CROP) {
     // CROP mode: Scale to fill screen completely (may crop edges)
     // Calculate crop values to fill the screen while maintaining aspect ratio