feat: Add group mode with per-user mute/volume controls

[journeyjt]

Features:

• Implement collaborative group mode for shared queue management
• Add per-user mute/volume preferences using client settings
• Enable non-DJ users to add songs to the queue in group mode
• Maintain individual audio preferences across session changes

Bug Fixes:

• Fix queue synchronization when DJ advances with non-DJ added videos (Bug 1)
• Fix clear queue operation to sync across all listeners and stop player (Bug 2)
• Fix mute state isolation to prevent DJ actions from affecting listener preferences (Bug 3)
• Fix mute button visual state synchronization with player state

Technical Improvements:

• Move mute/volume state from global to client-scoped settings
• Add localPlayerCommand hook for user-specific player operations
• Update YouTubePlayerWidget to sync visual states with async player operations
• Add comprehensive test coverage for group mode scenarios

Tests:

• Add GroupModeQueueOperations integration tests
• Add YouTubePlayerWidget mute button visual state tests
• Update existing tests for new client settings architecture