Delete custom exercises

[rd-code-23]

Summary

• add ownership-protected deletion for custom exercises
• confirm deletion from the Configure Exercise page
• remove deleted exercises from the TanStack Query cache and current routine draft
• return to the routine editor after successful deletion
• align Edit name and Delete exercise as nested actions beneath the custom exercise name
• show inline feedback when deletion fails

Note

PR #94 was merged into the feature branch it targeted rather than into main. This PR targets main and carries the deletion feature plus its final action layout into the application.

Verification

• focused API tests pass
• focused frontend tests pass
• API and web typechecks pass
• API and web lint pass