Restructure into a single PlatformIO project with nano and esp32 targets

- Shared BROSE9323 driver in lib/, ESPSERIAL selected via build_flags
- Nano firmware in src/nano, ESP32 firmware in src/esp32
- Move WiFi/MQTT credentials to gitignored secrets.h (template provided)
- Pin home-assistant-integration to 2.1.0 from the registry
- Add esp32_usb environment for first flash / recovery
- Add READMEs and CLAUDE.md

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

platformio.ini

@@ -0,0 +1,59 @@
+; Flipdot display - one PlatformIO project, two firmware targets.
+;
+;   nano       Arduino Nano that drives the Brose flipdot panels directly
+;   esp32      ESP32 "brain" (WiFi, NTP clock, Home Assistant) - OTA upload
+;   esp32_usb  Same ESP32 firmware, flashed over USB (first flash / recovery)
+;
+; Build all:        pio run
+; Build one:        pio run -e nano
+; Upload one:       pio run -e nano -t upload
+; Serial monitor:   pio device monitor -e nano
+;
+; https://docs.platformio.org/page/projectconf.html
+
+[platformio]
+default_envs = nano, esp32
+
+[env]
+framework = arduino
+monitor_speed = 115200
+lib_deps =
+	adafruit/Adafruit GFX Library@^1.10.12
+
+; ---------------------------------------------------------------------------
+; Arduino Nano - display driver
+; ---------------------------------------------------------------------------
+[env:nano]
+platform = atmelavr
+board = nanoatmega328
+board_build.mcu = atmega328p
+board_build.f_cpu = 16000000L
+build_src_filter = +<nano/>
+; Uncomment when using the PLCC adapter board (different pinout, see lib/BROSE9323/BROSE9323.h)
+; build_flags = -D FLIPDOT_PLCC_ADAPTER
+upload_port = COM10
+monitor_port = COM10
+
+; ---------------------------------------------------------------------------
+; ESP32 - controller, talks to the Nano over serial
+; ---------------------------------------------------------------------------
+[env:esp32]
+platform = espressif32
+board = esp32doit-devkit-v1
+build_src_filter = +<esp32/>
+; ESPSERIAL turns the BROSE9323 driver into a serial client: every draw call is
+; sent to the Nano as a text command instead of toggling GPIO pins.
+build_flags = -D ESPSERIAL
+lib_deps =
+	${env.lib_deps}
+	https://github.com/robjen/GFX_fonts.git
+	dawidchyrzynski/home-assistant-integration@^2.1.0
+	knolleary/PubSubClient@^2.8.0
+upload_protocol = espota
+upload_port = 10.1.1.198
+monitor_port = COM11
+
+[env:esp32_usb]
+extends = env:esp32
+upload_protocol = esptool
+upload_port = COM11