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/activities/boot_sleep/SleepActivity.h

@@ -1,10 +1,10 @@
 #pragma once
-#include "../Activity.h"
-
 #include <Bitmap.h>
 
 #include <string>
 
+#include "../Activity.h"
+
 class SleepActivity final : public Activity {
  public:
   explicit SleepActivity(GfxRenderer& renderer, MappedInputManager& mappedInput)