Add PN532 toolkit firmware, web UI, and embedded SPIFFS assets

Includes ESP-IDF NFC stack (deep capture, 4K Classic geometry, UL write API,
open SoftAP), React dashboard with live tag diagnostics, and docs. README
updated for APIs and lab Wi-Fi defaults.

Made-with: Cursor

docs/FLASHING.md

@@ -0,0 +1,34 @@
+# Flashing
+
+## Prerequisites
+
+- [ESP-IDF](https://docs.espressif.com/projects/esp-idf/en/latest/esp-idf-en-latest-esp32s3/esp32s3/get-started-esp32s3.html) **v5.x** installed and exported (`get_idf`)
+- USB cable to the DevKit **USB** port (USB‑JTAG / serial)
+- PN532 module wired per [PINOUT.md](PINOUT.md)
+
+## Steps
+
+1. Build the web assets (embeds into SPIFFS):
+
+   ```bash
+   cd web && npm install && npm run build:fw
+   ```
+
+2. Configure & flash:
+
+   ```bash
+   cd firmware
+   idf.py set-target esp32s3
+   idf.py menuconfig   # set PN532 transport + GPIOs
+   idf.py build flash monitor
+   ```
+
+3. Connect to AP **PN532-Toolkit** / **nfc-toolkit**, browse to **http://192.168.4.1** or **http://pn532tool.local**.
+
+## SPIFFS / UI missing?
+
+If `firmware/data/` is empty, the device serves a placeholder HTML. Always run `npm run build:fw` before `idf.py build` if you changed the UI.
+
+## Flash size
+
+`partitions.csv` assumes **8MB** flash. For **4MB**, shrink `factory` / OTA / `storage` regions and disable dual OTA if needed.