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>

src/esp32/libs.h

@@ -0,0 +1,71 @@
+#include <Arduino.h>
+#include "BROSE9323.h"
+#include "bitmaps.h"
+
+// WiFi / MQTT credentials (gitignored, copy secrets.example.h -> secrets.h)
+#if __has_include("secrets.h")
+#include "secrets.h"
+#else
+#error "Missing src/esp32/secrets.h - copy src/esp32/secrets.example.h to secrets.h and fill it in"
+#endif
+
+// fixes
+#include <SPI.h>
+#include <Adafruit_I2CDevice.h>
+// fonts
+#include <Fonts/FreeSerif12pt7b.h>
+#include <Fonts/FreeSerifItalic12pt7b.h>
+// #include <Font2x5FixedMonoNum.h>
+
+#include <WiFi.h>
+#include <ESPmDNS.h>
+#include <WiFiUdp.h>
+#include <ArduinoOTA.h>
+#include <WiFiMulti.h>
+
+// wdt?
+#include <esp_task_wdt.h>
+#include <rom/rtc.h>
+
+// hassio
+#include <ArduinoHA.h>
+
+byte mac[] = {0x34, 0x86, 0x5d, 0x3a, 0x08, 0x28};
+
+// EthernetClient client;
+WiFiClient client;
+HADevice device(mac, sizeof(mac));
+HAMqtt mqtt(client, device);
+
+HASwitch flipDotDisplay("FlipDotDisplayClock");
+bool hassioDisplayStatus = true;
+
+HASwitch powerSwitch("FlipDotDisplayPower");
+bool power = true;
+
+const int led1 = BUILTIN_LED; // Pin of the LED
+const int relay = 13; // Pin of the relay
+
+int hour = 0;
+int minute = 0;
+int second = 0;
+
+//
+WiFiMulti wifiMulti;
+
+const char *ssid = WIFI_SSID;
+const char *password = WIFI_PASSWORD;
+
+BROSE9323 display(84, 19, 28);
+
+uint32_t next_display;
+
+// ntp
+#define MYTZ "CET-1CEST,M3.5.0,M10.5.0/3"
+const char *ntpServer = "pool.ntp.org";
+const long gmtOffset_sec = 3600;
+const int daylightOffset_sec = 3600;
+// const int daylightOffset_sec = 0;
+
+bool wifiStatus = false;
+bool otaPanicMode = true;