feat: Configurable ERG mode resistance smoothing

[MaximumTrainer]

Summary

Add a configurable rolling-average smoothing applied to ERG setpoint commands sent to the trainer, reducing the jarring resistance steps that occur when transitioning between intervals with very different power targets.

Motivation

A hard step from, say, 150 W to 350 W causes a mechanical jolt and a brief period where the trainer briefly over- or under-shoots target. Ramping the resistance command over a user-configurable duration (e.g. 5 s) creates a smooth transition that is physically easier and reduces drivetrain stress.

Acceptance Criteria

• A ERG transition ramp duration setting (0 s = off, 2 s, 5 s, 10 s; default: 5 s) is present in the Trainer preferences tab
• When the ramp is active, setLoad() is called on every timer tick during the ramp period with linearly-interpolated wattage between old and new targets
• Ramp is applied only at interval transitions, not during steady-state ERG adjustments
• The ramp is skipped for rest-to-work transitions shorter than the ramp duration (i.e. ramp duration is clamped to the interval duration)
• The real-time power target display reflects the ramping target so the rider can see progress
• Setting ramp to 0 s disables the feature and reverts to the original step-change behaviour
• Unit tests cover linear interpolation correctness, clamping, and the 0-second passthrough case