feat: Auto-pause workout on sensor dropout

[MaximumTrainer]

Summary

Automatically pause the workout timer and ERG setpoint when the primary power or heart-rate sensor drops out for more than a configurable number of seconds, and resume automatically when the signal recovers.

Motivation

Currently a sensor dropout causes workout time to elapse with no data recorded and power targets remaining active on the trainer. Auto-pause prevents wasted training time and avoids incorrect workout metrics caused by silent data gaps.

Acceptance Criteria

• A configurable sensor dropout timeout (default: 5 s, range: 2–30 s) is exposed in the Trainer preferences tab
• When power (and/or HR, configurable) is absent for longer than the timeout, the workout is automatically paused and a dismissable overlay displays ''Sensor signal lost — workout paused''
• ERG load commands stop being sent while auto-paused
• When the sensor signal recovers, the workout resumes automatically after a 3-second countdown displayed on the overlay
• Auto-pause events are logged in the FIT file as pauses (identical to a manual pause)
• The auto-pause toggle can be disabled entirely in Preferences (e.g. for heart-rate-only sessions)
• Behaviour works identically in both ERG and slope modes
• Unit tests cover the dropout detection logic at boundary conditions (exactly at timeout, just below timeout)