Polish import/settings UI and fix Share Importer composer

[dprodger]

Summary

UI polish across the import/settings flows plus a fix for composer data not coming through when importing a song from MusicBrainz via the Share Importer.

Settings (logged-out)

• Replace the custom Sign In button with the standard ApproachNoteButton (no icon, primary styling); relabel to Sign In / Sign Up so it fits on one line.

Import Song shelf (iOS + macOS share extensions)

• Remove the music-note icon and the grey card container.
• De-emphasize the MusicBrainz ID (smaller monospace, secondary color).
• Widen the data fields to use the full available width.

Create Song form (iOS + macOS)

• Remove the MusicBrainz ID field and the entire Additional Details section (incl. the "added later through the app" footer).
• Move Cancel below Create Song, both using standard ApproachNoteButton styling.
• Drop the now-unused key state var. (Imported musicbrainz_id / work_type are still submitted to the API.)

Share Importer composer fix

The composer was never being scraped: on a MusicBrainz work page, composer is a relationship (<dt>composer:</dt><dd>…), not a classed sidebar property, so the old dd.composer/dd.writer selectors never matched. The work page is now also served behind a JS challenge, making DOM scraping unreliable.

• Resolve composers via the MusicBrainz web service (/ws/2/work/{id}?inc=artist-rels&fmt=json, same-origin) instead of the DOM.
• Prefer composer, fall back to writer/lyricist.
• Keep DOM scraping as a fallback; guard completionFunction to fire exactly once, with a 4s timeout safety net.

Test plan

• Build iOS and macOS schemes (SwiftUI + extension changes were not compiled locally).
• Re-import "Donna" (227437b0-…) and confirm composer populates as Jackie McLean (log should show composer = 'Jackie McLean').
• Verify Settings, Import Song shelf, and Create Song form render correctly on both platforms.

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