Refactor rest time storage from seconds to human-readable strings

[benmosher]

Summary

This PR refactors how rest times are stored in workout groups, changing from numeric seconds (restSeconds: number) to human-readable strings (rest: string). This improves data clarity and eliminates the need for parsing/formatting logic in the UI layer.

Key Changes

• Type definition update: Changed MovementGroup.restSeconds?: number to MovementGroup.rest?: string in workout-types.ts

• UI simplification: Removed parseRestSeconds() and formatRestSeconds() functions from WorkoutEditor.tsx and simplified the rest input handling to store/display raw string values

• Migration logic: Added migrateRest() and migrateRestSeconds() functions in plate-db.ts to handle backward compatibility with existing workouts that use the old restSeconds numeric format

• Export format update: Changed ExportedGroup.restSeconds?: number to ExportedGroup.rest?: string in workout-export.ts and updated packing/unpacking logic accordingly

• Viewer simplification: Removed formatRest() function from WorkoutViewer.tsx and updated display to use the rest string directly

• Importer update: Updated WorkoutImporter.tsx to work with the new rest string format

• Test updates: Updated test expectations in workout-export.test.ts to reflect the new string-based rest format

Implementation Details

• The migration logic automatically converts old numeric restSeconds values to human-readable strings (e.g., 180 → "3min", 90 → "1:30")
• Rest values are now stored as-is without parsing, reducing complexity and potential for formatting inconsistencies
• The change maintains backward compatibility by detecting and migrating old data during load

https://claude.ai/code/session_01Rz6TMvT8FwbYhcfFEWaDmU