feat: Proactive BLE sensor battery-level warnings

[MaximumTrainer]

Summary

Surface a proactive battery-level warning when any connected BLE sensor's battery drops below a configurable threshold, so users have time to swap batteries before starting a workout.

Motivation

The app already receives battery status notifications via batteryStatusReceived() in WorkoutDialog, but there is no proactive alert when the battery is low — the user must notice the passive indicator. A low-battery warning prevents mid-workout dropouts caused by depleted sensors.

Acceptance Criteria

• A configurable battery warning threshold (default: 20%, range: 5–50%) is present in the Trainer preferences tab
• When a sensor reports a battery level at or below the threshold, a dismissable toast or overlay is displayed listing the affected sensor type and battery level (e.g. ''Heart Rate sensor battery: 15%'')
• The warning fires: (a) immediately on connection if the reported level is already below threshold, and (b) at the moment the level crosses the threshold during a session
• The warning does not re-appear for the same sensor within the same session unless the battery level drops a further 5%
• Battery warnings are compatible with all supported sensor types: HR, cadence, speed, power, combined speed/cadence, oxygen
• Battery status is shown persistently in a small indicator (coloured dot) in the sensor pairing area throughout the workout
• Unit tests cover threshold boundary logic (at threshold, just above, just below) and suppression of repeated alerts