Split NETWORK_RADIO_CONFIG into individual env vars and add FEATURE_RADIO_CONFIG flag

- Replace single NETWORK_RADIO_CONFIG comma-delimited string with six
  individual environment variables: NETWORK_RADIO_PROFILE, _FREQUENCY,
  _BANDWIDTH, _SPREADING_FACTOR, _CODING_RATE, _TX_POWER
- Radio config fields now use raw numeric types (float/int) with units
  applied dynamically via RadioConfig.format_for_display()
- Add FEATURE_RADIO_CONFIG feature flag to control radio config panel
  visibility on the home page (default: enabled)
- Remove from_config_string class method (no backwards compatibility)
- Update Click CLI options, create_app() signature, and _build_config_json()
- Update docker-compose.yml, .env.example, README.md, AGENTS.md
- Add upgrading.md v0.12.0 section with migration instructions
- Add test coverage for schema, config, and feature flag

.env.example

@@ -420,10 +420,14 @@ NETWORK_NAME=MeshCore Network
 NETWORK_CITY=
 NETWORK_COUNTRY=
 
-# Radio configuration (comma-delimited)
-# Format: <profile>,<frequency>,<bandwidth>,<spreading_factor>,<coding_rate>,<tx_power>
-# Example: EU/UK Narrow,869.618MHz,62.5kHz,SF8,CR8,22dBm
-NETWORK_RADIO_CONFIG=
+# Radio configuration (six individual variables — no unit suffixes needed)
+# Units (MHz, kHz, dBm) are applied automatically on display
+NETWORK_RADIO_PROFILE=EU/UK Narrow
+NETWORK_RADIO_FREQUENCY=869.618
+NETWORK_RADIO_BANDWIDTH=62.5
+NETWORK_RADIO_SPREADING_FACTOR=8
+NETWORK_RADIO_CODING_RATE=8
+NETWORK_RADIO_TX_POWER=22
 
 # Welcome text displayed on the homepage (optional, plain text)
 # If not set, a default welcome message is shown
@@ -448,6 +452,7 @@ NETWORK_ANNOUNCEMENT=
 # FEATURE_MEMBERS=true
 # FEATURE_PAGES=true
 # FEATURE_CHANNELS=true
+# FEATURE_RADIO_CONFIG=true
 
 # -------------------
 # Contact Information