[SYS] Bound remaining MQTT/HTTP/BLE/RF inputs into fixed buffers (#2324)

Follow-up to #2321. That PR fixed five strcpy sites in XtoSYS/createOrUpdateDevice
but left adjacent attack surfaces using the same pattern (strncpy with the
destination's own size, leaving the buffer non-NUL-terminated when the source
is at the limit; or strcpy of WiFiManager/HTTP form values that bypass the
client-side maxlength). The over-long string then flows into saveConfig() and
back into a strcpy on next boot, corrupting heap/.bss on every reload.

main/main.cpp - XtoSYS: length-check before copying mqtt_topic, discovery_prefix,
  gateway_name, gw_pass (ota_pass), and ble_aes. Reject over-length with
  THEENGS_LOG_WARNING, preserving existing values.
main/main.cpp - loadConfigFromFlash: new loadStrField() helper applies the same
  guard to every SPIFFS-loaded string (mqtt_server/port/user/pass per slot,
  mqtt_topic, discovery_prefix, gateway_name, ota_pass, ble_aes). Defense in
  depth in case a pre-fix build left an over-length value in /config.json.
main/main.cpp - WiFiManager save: bound mqtt_topic against the 150-byte portal
  buffer overflowing the 66-byte destination (HTML maxlength is client-side
  only; a raw POST bypasses it).
main/webUI.cpp - handleCG /gw save: bound gateway-password form input before
  the strncpy that previously left ota_pass non-NUL-terminated.
main/blufi.cpp - ESP_BLUFI_EVENT_RECV_STA_SSID/PASSWD: clamp the wire-supplied
  ssid_len/passwd_len before the memcpy and the trailing NUL so a crafted
  BluFi provisioning frame cannot overrun gl_sta_ssid[32]/gl_sta_passwd[64].
main/gatewayBT.cpp - BLEDecryptor (Victron, encr==3): remove `nonce[16] = {0}`,
  a one-byte stack-OOB write reachable from any BLE adv. The line was dead
  (the memset two lines later zeroes the whole array); deleting it eliminates
  the OOB write.
main/gatewaySRFB.cpp - SRFBtoX: bound the _uartbuf writer so a 433 MHz frame
  with no STOP can no longer let _uartpos grow past sizeof(_uartbuf).
main/gatewaySRFB.cpp - _rfbSend: the hex output buffer was sized RF_MESSAGE_SIZE
  (9) but _rawToHex writes 2*N+2 bytes per call. Resize to fit.

Tested on a Theengs Bridge: built theengs-bridge-v11 env, OTA-flashed, and
verified that publishing {"discovery_prefix":"Z"*200} to MQTTtoSYS/config
leaves discovery_prefix unchanged (no Z*65 truncation in subsequent discovery
topics), bridge keeps decoding BLE adverts, and SYStoMQTT/LWT stay healthy.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

main/webUI.cpp

@@ -880,8 +880,17 @@ void handleCG() {
     }
     if (server.hasArg("save")) {
       if (server.hasArg("gp") && server.arg("gp").length() > 0 && strcmp(ota_pass, server.arg("gp").c_str())) {
-        strncpy(ota_pass, server.arg("gp").c_str(), parameters_size);
-        pwUpdate = true;
+        // HTML maxlength is client-side only — a direct POST can deliver any
+        // length. Reject anything that would not fit (and leave dst
+        // NUL-terminated) before copying into the fixed-size buffer.
+        const char* gp = server.arg("gp").c_str();
+        // ota_pass is extern char[] in this TU; size is parameters_size.
+        if (strlen(gp) >= parameters_size) {
+          THEENGS_LOG_WARNING(F("[WebUI] gateway password too long, ignoring" CR));
+        } else {
+          strcpy(ota_pass, gp);
+          pwUpdate = true;
+        }
       }
 #    ifdef ZmqttDiscovery
       if (SYSConfig.discovery != server.hasArg("dc")) {