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/marquee_demo.cpp.txt

@@ -0,0 +1,42 @@
+#include <Arduino.h>
+#include "BROSE9323.h"
+// fixes
+#include <SPI.h>
+#include <Adafruit_I2CDevice.h>
+
+BROSE9323 display(84, 19, 28);
+
+/*
+   UMLAUT:
+   Ä \x8E
+   ä \x84
+   Ö \x99
+   ö \x94
+   Ü \x9A
+   ü \x81
+   ß \xE0
+*/
+// const char text[] = "This is a test. Testing \x9Aml\x8Aute. ";
+const char text[] = "This is a test. Testing Ümläute. ";
+const uint8_t textsize = 2;
+
+void setup()
+{
+  display.begin();
+  delay(2000);
+  display.fillScreen(0);
+  display.setTextSize(textsize);
+  display.setTextWrap(false);
+  display.setTextColor(1, 0);
+  //display.setDirect(true); //when using setTextColor(1, 0) we don't need a separate display() and the text scrolls more evenly (on *328*)
+}
+
+void loop()
+{
+  for (int16_t x = display.width(); x > -((int16_t)strlen(text) * textsize * 6); x--)
+  {
+    display.setCursor(x, 0);
+    display.print(text);
+    display.display();
+  }
+}