chore: Add Australia (Mid) Radio Preset

This mirrors the config option from the official Meshcore app, and represents the preset now commonly in use in New South Wales.

lib/models/radio_settings.dart

@@ -81,6 +81,16 @@ class RadioSettings {
         txPowerDbm: 20,
       ),
     ),
+    (
+      'Australia (Mid)',
+      RadioSettings(
+        frequencyMHz: 915.075,
+        bandwidth: LoRaBandwidth.bw125,
+        spreadingFactor: LoRaSpreadingFactor.sf9,
+        codingRate: LoRaCodingRate.cr4_5,
+        txPowerDbm: 20,
+      ),
+    ),
     (
       'Australia SA, WA, QLD',
       RadioSettings(