TCP scaling, GUI pooling, dev mode, protocol improvements

- TCP: wider drain budgets, TCP_NODELAY, no flush, PSRAM buffers, queue announces until hub ID
- GUI: object pool Messages/Contacts screens, partial MessageView status, targeted tab refresh
- LVGL throttled to 5fps when dimmed, non-blocking RSSI monitor, bounded I2S writes
- LXMF batch drain, BLE frame mutex, LoRa interrupt-driven RX, name cache cap 300
- Developer mode in settings: custom radio params (freq/txp/SF/BW/CR/preamble) behind warning

src/config/UserConfig.cpp

@@ -59,6 +59,7 @@ bool UserConfig::parseJson(const String& json) {
     _settings.audioVolume  = doc["audio_vol"] | 80;
 
     _settings.displayName = doc["display_name"] | "";
+    _settings.devMode     = doc["dev_mode"]     | false;
 
     Serial.println("[CONFIG] Settings loaded");
     return true;
@@ -100,6 +101,7 @@ String UserConfig::serializeToJson() const {
     doc["audio_vol"] = _settings.audioVolume;
 
     doc["display_name"] = _settings.displayName;
+    doc["dev_mode"]     = _settings.devMode;
 
     String json;
     serializeJson(doc, json);