Confirm before deleting clips used in arrangement

[britle20]

Bug

Deleting an otherwise-empty hybrid clip that is currently placed in the arrangement does not ask for confirmation, even though the delete operation also removes its arrangement clip instances.

Steps to reproduce

1. Create or select a hybrid clip with no drum steps or piano notes.
2. Place the clip in the arrangement view.
3. Delete the source clip from the sidebar.

Expected behavior

The app should ask for confirmation before deleting a clip when any of these are true:

• The clip is an imported audio clip.
• The clip has drum events or piano roll note events.
• The clip has one or more arrangement clip instances.

Actual behavior

The app only confirms when the clip has musical data, or when it is an imported audio clip. An empty hybrid clip used in the arrangement can be deleted without confirmation.

Relevant files or docs

• src/app/App.tsx
• src/features/project-sidebar/ProjectSidebar.tsx
• docs/data-model.md

Done when

• Clip deletion confirmation includes arrangement usage.
• The confirmation message explains that arrangement placements will also be removed.
• Empty hybrid clips that are not used in the arrangement can still be deleted without confirmation.

Verification

• npm run typecheck --if-present
• npm run lint --if-present
• npm run test --if-present
• npm run build --if-present

Notes

This is a focused safety fix, not a new product feature spec.

[britle20]

Implemented by PR #56, merged into develop.