Harden web UI auth, input handling, and SD path validation

- Add optional Basic Auth with NVS-backed credentials and STA/AP flags; protect status, wifi, history, and download routes

- Stop pre-filling WiFi/MQTT/Web UI password fields; keep stored secrets on blank and add clear-password checkboxes

- Add HTML escaping + URL encoding helpers and apply to user-controlled strings; add unit test

- Harden /sd/download path validation (prefix, length, dotdot, slashes) and log rejections

- Enforce protocol version in LoRa receive and release GPIO14 before SD init

- Update README security, SD, and GPIO sharing notes

src/main.cpp

@@ -626,6 +626,7 @@ void setup() {
     update_battery_cache();
   } else {
     power_receiver_init();
+    pinMode(PIN_ROLE, INPUT); // release pulldown before SD uses GPIO14 as SCK
     sd_logger_init();
     wifi_manager_init();
     init_sender_statuses();