Add play actions for playlists

[Padma-Dev-E]

Summary

Adds playlist-level Play actions so users can start playback directly from playlist headers instead of playing tracks one by one.

This affects:

• Provider playlist screens
• Local Library playlist/folder screens

The Play action starts only available local/downloaded tracks in playlist order, disables shuffle before playback, and shows an error snackbar if no playable local file is available.

Related Issues

Fixes #490

Type of Change

• Bug fix
• Feature
• Performance improvement
• Refactor with no intended behavior change
• Tests, build, or repository maintenance
• Documentation or localization

Validation

• dart format --output=none --set-exit-if-changed lib test
• flutter analyze
• flutter test
• gofmt, go vet ./..., and go test ./... in go_backend/
• Android or iOS native checks
• Manual reproduction or device test

Validation notes:

Verified on iPhone 17 simulator. Confirmed the playlist header now shows Play next to Download All, and tapping Play starts playback for available local/downloaded playlist tracks in order.

Skipped flutter test because this change is UI/playback wiring and no targeted widget/unit test exists for this playlist header flow.

Skipped Go checks because this PR does not change go_backend/.

Contributor Checklist

• This PR is focused and contains no unrelated changes.
• Extension-specific behavior, if any, uses a generic capability or manifest contract.
• User-facing strings, documentation, and generated files are updated where needed.
• No credentials, downloaded media, or build artifacts are included.

Screenshots or Recordings

Add screenshot/video showing the playlist header with the new Play button next to Download All.