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>

archive/README.md

@@ -0,0 +1,8 @@
+# Archive
+
+Old sketches kept for reference. They are **not** built. The `.txt` extension keeps
+PlatformIO from compiling them.
+
+- `SerialAPI_nano_old.cpp.txt`: an earlier version of the Nano serial firmware, from
+  before the EEPROM timing and the default flip time were added.
+- `marquee_demo.cpp.txt`: a scrolling text (marquee) demo, including an umlaut test.