feat(playground): add accessible context menus

[txbrown]

Summary

Web users can now manage tracks, sections, and clips with accessible controls instead of relying on native-only action sheets. The shared SongView supports click, keyboard, screen-reader, long-press, and browser context-menu entry points while preserving the native rule that the final track cannot be deleted.

What changed

• Added track, section, and clip menus with explicit accessible triggers and in-menu Close/Cancel actions.
• Added edit, duplicate, and delete actions that map directly to the shared song store contract.
• Kept modal backdrops outside the accessibility tree and isolated focus within each menu.
• Added stable indexed test IDs to playground dashboard menu triggers.
• Updated test stores, behavior coverage, and snapshots for the expanded action contract.

Validation

• 32 targeted tests passed across SongView, Mixer, and PlaygroundsDashboard.
• 14 snapshots passed.
• Circuit UI lint and TypeScript build checks passed.

Manual browser keyboard/context-menu and VoiceOver/TalkBack QA remains recommended.

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