Enable removing attachments in task form

[bborn]

Summary

• render attachments as selectable chips with keyboard removal
• add key handling so users can delete attachments before submitting
• theme-aware chip styles and regression test

Testing

• go test ./...

[openhands-ai]

Looks like there are a few issues preventing this PR from being merged!

• GitHub Actions are failing:
  • CI

If you'd like me to help, just leave a comment, like

@OpenHands please fix the failing actions on PR #279 at branch `task/576-need-to-be-able-to-remove-file-attachmen`

Feel free to include any additional details that might help me get this PR into a better state.

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