Add "Copy to Split" feature for mesocycle exercise configuration

[wulfland]

User Story

As a lifter,
I want to copy exercises from one split day to another when configuring a mesocycle,
So that I can quickly set up similar training days without manually adding each exercise again.

Background

When creating mesocycles with repeating split patterns (upper/lower, full body), lifters often want similar or identical exercise selections across matching days:

• Upper/Lower: Upper 1 and Upper 2 may share most exercises
• Full Body: All days typically include the same compound movements
• Push/Pull/Legs: Push 1 and Push 2, Pull 1 and Pull 2, etc.

Currently, users must manually add each exercise to every split day, which is time-consuming and error-prone.

Acceptance Criteria

• Add a "Copy to..." button/action in the split day panel header
• Show a dropdown or modal to select target split day(s)
• Allow copying all exercises from the current split to the selected target(s)
• If target already has exercises, prompt user to choose:
  • Replace existing exercises
  • Append to existing exercises
  • Cancel
• Preserve exercise order when copying
• Copy exercise configuration (sets, rep ranges if configured)
• Provide visual feedback on successful copy (toast notification)
• Works offline (local-only operation)

UI/UX Considerations

• Copy action should be easily discoverable but not clutter the interface
• Consider placing near the split tab or in a "..." menu for the panel
• Clear visual indication of source and target splits
• Undo capability would be nice-to-have

Technical Notes

• This is a core (free) feature - local-only operation
• No database changes required - operates on in-memory state during mesocycle configuration
• Should integrate with existing MesocycleExerciseConfig component
• Consider reusability for future "template" features

Out of Scope (for this issue)

• Copying individual exercises (drag & drop)
• Cross-mesocycle copying
• Saving split configurations as templates

Related

• Part of mesocycle creation/editing workflow
• Enhances exercise configuration UX