Add flash utility, display fixes, and comprehensive README

- flash.py: standalone flash utility with serial port listing, merge-bin,
  GitHub Releases download, and esptool flash support
- Display.h: hide LAN row when Local TCP disabled, show local TCP port
  instead of backbone port
- README.md: comprehensive documentation — Quick Start with 3 flash options,
  OLED display layout, interface modes, routing customizations, path table
  fix, interface name uniqueness, hardware rationale (PSRAM/flash)
- Release/boot_app0.bin: bundled for flash.py standalone use
- .gitignore: exclude merged firmware binary build artifact

.gitignore

@@ -2,6 +2,7 @@
 *.hex
 *.pyc
 TODO
+rnodethv4_firmware.bin
 Release/*.hex
 Release/*.zip
 Release/*.json