feat(workout): implement Power Match (Control ERG via external Power Meter)

[SoNdA11]

Description:
Many advanced cyclists prefer to use their on-bike Power Meter (pedals/cranks) as the "source of truth" rather than the trainer's internal estimation. Power Match allows the software to read power from one sensor and send resistance commands to another based on that reading.

Acceptance Criteria:

• Multi-Sensor Logic: Update the BLEService to allow simultaneous connections to a Smart Trainer (FTMS) and a standalone Power Meter.
• PID Controller: Implement a logic loop that adjusts the Trainer's target resistance dynamically to match the output reported by the external Power Meter.
• UI Integration: Add a toggle in the Device Connection screen to select which device provides the "Control Power".
• Ensure telemetry and .fit files record the chosen "source of truth" power.