feat: TCX and GPX export format selection on post-workout panel

[MaximumTrainer]

Summary

Allow users to export completed activities in TCX or GPX format from the post-workout panel in addition to the automatically-generated FIT file, providing compatibility with platforms that do not accept FIT (e.g. older Garmin devices, some third-party apps).

Motivation

A XmlStreamWriterTCX and GpxParser already exist in src/persistence/file/ but they are not exposed in the UI. Many training platforms and devices accept TCX or GPX but not FIT, so exposing these existing writers directly is a low-effort, high-value improvement.

Acceptance Criteria

• The post-workout panel includes an Export section with three buttons: ''Save as FIT'' (auto-saved, greyed out once done), ''Save as TCX'', ''Save as GPX''
• Clicking ''Save as TCX'' or ''Save as GPX'' opens a file-save dialog pre-populated with the workout name and date
• The TCX export uses the existing XmlStreamWriterTCX and produces a valid TCX 2.0 file
• The GPX export uses GpxParser (or a new GpxWriter following the same pattern) and produces a valid GPX 1.1 file with power, HR, and cadence extensions (Garmin TrainingCenterDatabase extension)
• Export is disabled if no DataWorkout object is available (i.e. workout was cancelled before any data was recorded)
• On WASM, the export triggers a browser file download rather than a native save dialog
• Unit tests verify TCX and GPX output against a reference file for a known DataWorkout fixture
• Existing auto-save-as-FIT behaviour is unchanged