Add LoRa coding rate preset override controls

[RCGV1]

Summary

• Adds a LoRa coding-rate control to the LoRa config screen.
• Keeps preset mode on preset bandwidth/spreading factor while allowing only firmware-valid higher coding-rate overrides.
• Uses a default toggle plus stepped slider so users can see when they are following the preset and when they are adding redundancy.
• Preserves protobuf default behavior by saving coding_rate = 0 for preset-default CR instead of writing an explicit equal-to-default value.

Reasoning

Firmware now allows config.lora.coding_rate to override a preset only when the value is a valid higher/more redundant CR. Equal or lower values should fall back to the preset CR. The UI mirrors that rule by filtering preset-mode choices to 0 or strictly higher values, while custom mode still exposes explicit 4/5...4/8 values.

Screenshot

Verification

• git diff --check passed.
• Built for Benjamin's iPhone with:
  xcodebuild -workspace Meshtastic.xcworkspace -scheme Meshtastic -configuration Debug -destination 'platform=iOS,id=00008130-000578880102001C' -derivedDataPath /tmp/meshtastic-device-derived -skipPackagePluginValidation -skipMacroValidation build
• Initial device build hit a codesign internal error while disk was nearly full; after clearing this branch's derived-data caches, the same build succeeded.
• Installed the built app to the iPhone successfully. Launch was denied because the phone was locked.
• Focused test compile was attempted earlier, but the test bundle hit the same local code-signing/disk-space issue rather than a source failure.

[garthvh]

I don't really like this pattern, once you go to custom stuff you are customizing things.

[RCGV1]

I disagree, modifying your coding rate isn't "custom" it doesn't make your packets get ignored by other nodes running different coding rates. I want to encourage people to use different coding rates, as depending on your deployment, you may need a higher coding rate to get out. @NomDeTom can back me up here