Add unified library endpoint parity

[guidefari]

Parent PRD

#9

What to build

Add support for Spotify's unified /me/library endpoints so consumers can save, remove, and check items through the modern generic API in addition to the existing type-specific helpers. Keep the slice focused on the end-to-end public API shape and request behavior.

Acceptance criteria

• Public library service supports unified save, remove, and contains operations backed by /me/library
• Request options and payloads match Spotify's generic library endpoint semantics
• Tests cover observable behavior for the unified library operations

Blocked by

None - can start immediately

User stories addressed

• User story 16
• User story 17
• User story 29
• User story 37
• User story 40

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Migrated from guidefari/spotify-effect#58