fix: prevent Serial.printf from blocking when USB disconnected

On ESP32-C3 with USB CDC, Serial.printf() blocks indefinitely when USB
is not connected. This caused device freezes when booted without USB.

Solution: Call Serial.setTxTimeoutMs(0) after Serial.begin() to make
all Serial output non-blocking.

Also added if (Serial) guards to high-traffic logging paths in
EpubReaderActivity as belt-and-suspenders protection.

Includes documentation of the debugging process and Serial call inventory.

Also applies clang-format to fix pre-existing formatting issues.

src/BookManager.cpp

@@ -35,9 +35,7 @@ std::string BookManager::getExtension(const std::string& path) {
   return ext;
 }
 
-size_t BookManager::computePathHash(const std::string& path) {
-  return std::hash<std::string>{}(path);
-}
+size_t BookManager::computePathHash(const std::string& path) { return std::hash<std::string>{}(path); }
 
 std::string BookManager::getCachePrefix(const std::string& path) {
   const std::string ext = getExtension(path);