feat: Multi-Setlist Management

[ElBeh]

Description

Support multiple named setlists per song library. Users can create, switch,
rename, duplicate, and delete setlists via a dropdown in the sidebar setlist
header. Prepares the data model for a future "Gig" grouping layer
(Gig → Setlists → Songs).

Estimated effort: ~6–8h

Requirements

• Dropdown in sidebar setlist header to switch between saved setlists
• Create new setlist (inline name input)
• Rename active setlist
• Duplicate setlist (copy with "(Copy)" suffix)
• Delete setlist (with confirmation, prevent deleting the last one)
• Mark one setlist as default (auto-loaded on app start)
• Display total duration (sum of song durations + pause durations) in setlist header
• Context action on song entry: "Copy to Setlist X" / "Move to Setlist X"
• When deleting a song from the library: warn if referenced in other setlists
• Songs remain in library; setlists hold references (song IDs + order)
• Band Sync: sync active setlist selection to viewers (new sync event)

Implementation

• New IndexedDB store setlists with schema { id, name, items: SetlistItem[], isDefault: boolean }
• DB version bump (7 → 8) with migration: wrap existing songOrder/setlistEntries config values into a default setlist entry
• Consider extracting useSetlistStore from useSongStore (currently manages songOrder + setlistEntries)
• Dropdown component: plain <select> or custom Tailwind dropdown, no external UI library
• New multi-setlist export format: { setlists: [{ name, items[] }, ...], songs: [...] }
• Backward-compatible import: detect old single-setlist format and auto-migrate
• Add setlist:switch event to syncProtocol.ts

Decisions

• Dedicated IndexedDB store over config key-value: setlists are a first-class entity with their own lifecycle, not a setting
• Export format designed to be forward-compatible with future Gig-level grouping (Gig wraps multiple setlist IDs) — no breaking change needed later
• useSetlistStore extraction keeps domain separation clean per CONVENTIONS.md (one store per domain)

Out of Scope

• Gig-level grouping (separate issue once multi-setlist is stable)
• Setlist sharing between devices outside of Band Sync
• Setlist templates/presets