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/nano/README.md

@@ -0,0 +1,57 @@
+# Arduino Nano: flipdot driver
+
+PlatformIO environment: `nano`
+
+The Nano is connected directly to the Brose panel electronics. It keeps an 84 × 19 frame
+buffer and takes drawing commands over serial (115200 baud). It has no clock or network
+logic.
+
+## Behaviour
+
+- **Boot:** the Nano flips the whole display on and then off, shows the splash bitmap and
+  prints the time a full redraw took.
+- **`display()`:** compares the new buffer with the last one that was shown and strobes
+  only the dots that changed.
+- **Every 2 minutes** (`REFRESH_TIME`) it forces a full redraw to fix dots that got stuck.
+- **Flip pulse length** (µs) is set with the `T` command and stored in EEPROM, so it is
+  kept after a reboot.
+
+## Serial protocol
+
+Each command is one ASCII line. Coordinates are 2-digit **hex**. Color is `0` (black) or
+`1` (yellow). The ESP32 driver (`ESPSERIAL`) only uses `s`, `S`, `F`, `D` and `T`.
+
+| Cmd | Format | Meaning |
+|-----|--------|---------|
+| `s` | `sXXYYc` | Set pixel in the buffer |
+| `S` | `SXXYYc` | Set pixel and flip it right away (direct mode) |
+| `F` | `Fc` | Fill the whole buffer |
+| `D` | `D` | Push the buffer to the display (changed dots only) |
+| `P` | `PXXYYcsTEXT` | Print text at (XX,YY), color `c`, size `s`. `\n` in TEXT starts a new line. |
+| `B` | `BXXYYWWHHcHEX...` | Draw a bitmap. WW must be a multiple of 4. Pixel data is one hex digit per 4 pixels. |
+| `R` | `R` | Print the buffer as ASCII art on the serial port |
+| `T` | `Tnnn` | Set the flip pulse length in µs (decimal), stored in EEPROM |
+
+Example (what the ESP32 sends for one pixel and a refresh):
+
+```
+s0a051
+D
+```
+
+## Wiring (default pinout)
+
+The pins are defined in [lib/BROSE9323/BROSE9323.h](../../lib/BROSE9323/BROSE9323.h).
+If you use the PLCC adapter board, build with `-D FLIPDOT_PLCC_ADAPTER` to switch to its
+pinout.
+
+| Signal | Nano pin |
+|--------|----------|
+| ENABLE | D7 |
+| ADDR_0 / 1 / 2 (panel select) | D3 / D5 / D6 |
+| COL_DATA | D4 |
+| COL_0 … COL_4 | D8, D10, D9, D11, D12 |
+| ROW_0 … ROW_4 | A3, A4, A2, A0, A1 |
+| ROW_RESET | D13 |
+| ROW_SET | A5 |
+| Serial RX | D0 (from ESP32 TX) |