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.

.gitignore

@@ -11,6 +11,7 @@ build
 **/__pycache__/
 test/epubs/
 CrossPoint-ef.md
+Serial_print.code-search
 
 # Gitea Actions runner config (contains credentials)
 .runner