Add a new frequency range for EU SRD area with adaptive DC

[caveman99]

We might wanna consider switching telemetry and maybe even position off for this band to operate with the reduced duty cycle.

[caveman99]

this requires a) meshtastic/protobufs#814 and b) further discussion

[Copilot]

Pull Request Overview

This PR adds support for the EU 866MHz RFID band (EU_866 region) with adaptive duty cycle enforcement based on device role. The implementation introduces a 2.5% duty cycle for mobile devices and 10% for fixed router devices, following ETSI EN 302 208 specifications.

Key changes:

• New EU_866 region definition with 4 channels at 865.7/866.3/866.9/867.5 MHz
• Adaptive duty cycle function that adjusts based on device role (router vs mobile)
• Enforcement of 125kHz bandwidth requirement with fallback to LiteFast preset
• Updated channel calculation logic to properly handle the 475kHz channel spacing

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/mesh/RadioInterface.cpp	Adds EU_866 region definition, implements getEffectiveDutyCycle() function, adds LITE_FAST preset support, enforces bandwidth restrictions, and improves channel calculation logic
src/mesh/MeshRadio.h	Declares getEffectiveDutyCycle() function with documentation
src/modules/AdminModule.cpp	Replaces direct dutyCycle access with getEffectiveDutyCycle() call
src/mesh/Router.cpp	Updates duty cycle enforcement to use getEffectiveDutyCycle()
src/graphics/draw/MenuHandler.cpp	Updates duty cycle check to use getEffectiveDutyCycle()
src/airtime.cpp	Updates airtime utilization checks to use getEffectiveDutyCycle()
src/DisplayFormatters.cpp	Adds display name support for LITE_FAST modem preset

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