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/reader/EpubReaderActivity.h

@@ -24,7 +24,7 @@ class EpubReaderActivity final : public ActivityWithSubactivity {
   // End-of-book prompt state
   bool showingEndOfBookPrompt = false;
   int endOfBookSelection = 2;  // 0=Archive, 1=Delete, 2=Keep (default to safe option)
-  
+
   // Content offset for position restoration after re-indexing
   uint32_t savedContentOffset = 0;
   bool hasContentOffset = false;  // True if we have a valid content offset to use