Allow creating new exercises while configuring mesocycle

[wulfland]

Summary

When adding exercises to a mesocycle's split days, users should be able to create a new exercise directly from the exercise selection interface, rather than having to navigate away to the Exercises page first.

Problem

Currently, when configuring a mesocycle and selecting exercises for a split day, users can only choose from existing exercises. If the exercise they want isn't in the library, they must:

1. Cancel or save the current mesocycle configuration
2. Navigate to the Exercises page
3. Create the new exercise
4. Navigate back to the mesocycle
5. Continue configuration

This interrupts the user's workflow and creates friction during mesocycle setup.

Proposed Solution

Add a "Create New Exercise" option in the exercise selection UI when configuring mesocycle split days. This could be:

• A button at the top or bottom of the exercise list
• An option that appears when searching yields no results
• A "+" icon next to the exercise dropdown/list

Clicking this should open the exercise creation form (could be a modal/dialog) and after saving, automatically add the new exercise to the current split day.

Acceptance Criteria

• Users can create a new exercise without leaving the mesocycle configuration flow
• The newly created exercise is automatically selected/added to the current split day
• The new exercise is saved to the exercise library for future use
• The exercise creation form includes all standard fields (name, category, muscle groups, equipment type)
• Works offline (exercise saved to IndexedDB)

Design Considerations

• Keep the inline creation simple - full exercise editing can still happen on the Exercises page
• Consider showing a "quick create" with just essential fields (name, category) vs full form
• Maintain consistency with the existing exercise form UI

[copilot-swe-agent]

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