Fixes device hanging when booted without USB connected. The root cause
was calling Serial.available() and Serial.read() in checkForFlashCommand()
when Serial.begin() was never called (USB not connected at boot).
Changes:
- Add if (!Serial) return guard to checkForFlashCommand()
- Restore upstream while (!Serial) wait loop with 3s timeout
- Remove Serial.setTxTimeoutMs(0) (not in upstream, may cause issues)
- Remove unnecessary if (Serial) guards from EpubReaderActivity.cpp
(Serial.printf is safe without guards, only input calls need them)
Key insight: Serial.printf() is safe without guards (returns 0 when not
initialized), but Serial.available()/Serial.read() cause undefined
behavior on ESP32-C3 USB CDC when called without Serial.begin().
See: claude_notes/usb-serial-blocking-fix-2026-01-28.md
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