Add factory reset functionality and update README instructions

README.md

@@ -77,6 +77,19 @@ After connecting, the device is reachable at:
 - `http://m1730.local` (mDNS, works on most local networks)
 - `http://<IP address>` (shown in the serial monitor on boot)
 
+### Factory reset (forgot password)
+
+If you lose the web UI password, hold the **BOOT button** (GPIO0) for 3 seconds during startup. The firmware resets all settings to factory defaults **except** meter pin assignments and calibration (maxDuty), which are preserved. On acknowledgement, all connected meters sweep 0→100→0.
+
+Serial output confirms the reset:
+
+```
+[BOOT] BOOT button held, hold 3s for factory reset...
+[BOOT] factory reset triggered!
+...
+[BOOT] factory reset complete, restarting...
+```
+
 ---
 
 ## Web UI
@@ -223,4 +236,4 @@ Config is stored as JSON in LittleFS at `/config.json`.
 }
 ```
 
-The file is written by the web UI and should not need manual editing. To reset to factory defaults, delete the file or erase flash with `pio run --target erase`.
+The file is written by the web UI and should not need manual editing. To reset to factory defaults (keeping pin/calibration), hold the BOOT button for 3 seconds at startup. To erase everything including flash, use `pio run --target erase`.