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/README.md

@@ -0,0 +1,45 @@
+# ESP32: controller
+
+PlatformIO environments: `esp32` (OTA) and `esp32_usb` (USB)
+
+The ESP32 decides what is shown on the display. It never touches the panel itself. The
+BROSE9323 driver is built with `-D ESPSERIAL`, so every GFX call is sent over `Serial`
+(115200 baud) to the Nano.
+
+## Files
+
+| File | Contents |
+|------|----------|
+| `main.cpp` | setup, OTA, clock / power logic, Home Assistant callbacks |
+| `libs.h` | includes and global objects (display, MQTT, WiFi, pins, timezone) |
+| `bitmaps.h` | 1-bit bitmaps (Pikachu, thought bubble, Nintendo logo, ...) |
+| `secrets.example.h` | template for `secrets.h` (WiFi and MQTT credentials, **gitignored**) |
+
+## Features
+
+- **OTA updates** through ArduinoOTA. Hostname: `FlipDotDisplay`.
+- **OTA panic window:** after a crash or watchdog reset, the ESP32 runs only the OTA
+  handler for 15 seconds before it starts the main code. A firmware that crashes on boot
+  can still be replaced over the air.
+- **NTP time** with timezone `CET-1CEST,M3.5.0,M10.5.0/3` (Europe/Amsterdam).
+- **Home Assistant** through MQTT (`home-assistant-integration` library) with two switches:
+  - `FlipDotDisplayClock`: clock on, or the Nintendo logo when off
+  - `FlipDotDisplayPower`: turns the display off through the relay
+- **Relay on GPIO 13** switches the display's power supply. `LOW` turns it on.
+- **Status LED** (built-in) blinks fast when WiFi is down and slowly when connected. It is
+  off at night or when the display is powered off.
+
+## Schedule
+
+| Time | Display |
+|------|---------|
+| 07:00 – 17:00 | Pikachu and the clock, updated every minute |
+| 17:00 – 22:00 | Yawning Pikachu and the clock |
+| 22:00 – 23:30 | Clock drawn only on the quarter hour; power is off in between |
+| night | Screen cleared, power off |
+
+## Adding bitmaps
+
+Convert an image with [image2cpp](https://javl.github.io/image2cpp/) to "Arduino code",
+horizontal, 1 bit per pixel. Paste the array into `bitmaps.h` and draw it with
+`display.drawBitmap(x, y, bitmap, w, h, 1)`. The display is 84 × 19 pixels.