iOS: per-group accordions in song Recordings section

[dprodger]

Summary

• Removed the outer expand/collapse chevron on the iOS song detail's Recordings section — the section is now always visible.
• Each grouping shelf (decade when sorted by year, artist name when sorted by name) is now its own accordion row that starts collapsed, so first view of a song shows a scannable list like "1960s (12)" / "1970s (8)" or "Miles Davis (4)" / "John Coltrane (3)" with items hidden until tapped.
• Per-group expansion state is tracked in a Set<String> keyed by group name; switching sort order clears it (since decade keys and artist-name keys don't overlap), filter changes preserve it for any surviving keys.

Test plan

• Open a song with many recordings on iOS — Recordings header has no chevron, list of decade/artist shelves all collapsed by default.
• Tap a shelf — it expands to show the horizontal scroll of recording cards; tap again to collapse.
• Apply / remove a filter while a shelf is open — surviving shelves stay in their previous open/closed state; newly added shelves are collapsed.
• Switch sort order (Year ↔ Name) — all shelves reset to collapsed.
• Verify the "Reloading..." overlay still shows during sort-driven reloads.

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