Allow adding without an arrangement (currently the UI requires picking one)

[mogul]

Restored from slopsmith/slopsmith-plugin-setlist#3 — original issue, opened by @byrongamatos on 2026-05-09.
[restored-from: slopsmith/slopsmith-plugin-setlist#3]

From specs/001-setlist/tasks.md — generated retrospectively by spec-kit.

Allow adding without an arrangement (currently the UI requires picking one).

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See specs/001-setlist/spec.md and specs/001-setlist/analyze.md for context.
This issue was filed in bulk; close as wontfix if no longer relevant.