feat: Workout queue / back-to-back playlist

[MaximumTrainer]

Summary

Allow users to queue multiple workouts to play sequentially without returning to the main window between sessions — a ''playlist'' mode useful for training camps, back-to-back block sessions, or custom warm-up + main + cool-down sequences.

Motivation

Currently completing a workout always returns to the main window, breaking flow for multi-session blocks. A queue lets athletes build compound sessions (e.g. 10-min warm-up → main interval set → 10-min cool-down) and chain them without manual interaction.

Acceptance Criteria

• A Queue button on the workout list adds a workout to the end of the current queue
• A Queue panel (side drawer or overlay) shows the ordered list of queued workouts and allows drag-to-reorder and removal
• When a workout finishes, the post-workout panel shows a countdown (default: 60 s, configurable) before the next workout starts automatically; the user can skip immediately or cancel the queue
• Total estimated queue duration and cumulative TSS are shown in the queue panel
• The FIT file for each workout in the queue is saved separately
• The queue is persisted to disk so it survives an application restart
• An empty queue reverts to standard single-workout behaviour
• Integration tests cover: queue ordering, auto-advance, countdown cancellation, and queue serialisation/deserialisation