feat: Per-interval transition summary overlay

[MaximumTrainer]

Summary

When an interval ends (or when the user manually advances), display a brief interval summary overlay inside the workout player showing key metrics for the just-completed interval before the next one begins.

Motivation

Riders benefit from immediate per-interval feedback (avg power, avg HR, avg cadence, TSS) at each transition. Currently this data is only available post-workout in the saving window. In-the-moment feedback allows adjustments (e.g. pacing, gear choice) for subsequent intervals.

Acceptance Criteria

• On interval transition, a translucent overlay appears in the workout plot area for a configurable duration (default: 5 s, range: 2–15 s, configurable in Preferences)
• The overlay displays for the completed interval: avg power (W and % FTP), avg HR (bpm and % LTHR), avg cadence (rpm), interval duration, and interval TSS
• If the target was power-based, a colour indicator (green / amber / red) shows whether the power target was met (within ±5%, within ±10%, or outside ±10%)
• The overlay can be dismissed early by pressing any key or clicking it
• The overlay does not appear if the interval was shorter than 10 s (avoids flicker on rapid manual advances)
• The feature can be disabled entirely in Preferences
• Layout does not interfere with the main power/HR plot or the timer widgets
• Unit tests cover target-achievement colour classification at boundary values