[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/gatewayBT.cpp

@@ -1348,7 +1348,10 @@ void process_bledata(JsonObject& BLEdata) {
       THEENGS_LOG_TRACE(F("[BLEDecryptor] BTHomeV2 nonce %s" CR), NimBLEUtils::dataToHexString(nonce, noncelength).c_str());
 
     } else if (BLEdata["encr"].as<int>() == 3) {
-      nonce[16] = {0}; // Victron has a 16 byte zero padded nonce with IV bytes 6,7
+      // Victron has a 16-byte zero-padded nonce with IV bytes 0,1.
+      // (The line `nonce[16] = {0};` that lived here wrote a byte one past the
+      // end of the 16-byte `nonce` array on every Victron-encrypted advert;
+      // memset below already zeroes the whole buffer, so the typo was dead.)
       unsigned char iv[2];
       int ivlen = hexToBytes(BLEdata["ctr"].as<String>(), iv, 2);
       if (ivlen != 2) {