[gefydev]

Description

• Audits and removes all remaining mock data across all UI views and components.
• Replaces hardcoded fallback playlists ('Synthwave Architect', 'Rustaceans Unite', 'Chill Lofi Beats') with animated skeleton rows during loading.
• Eliminates dead sidebar_item implementation.
• Wires top bar user avatar to OpenSpotifyAccount and plus button to toast notification.
• Connects playback bar Heart button to ShowToast and queue panel current item to TogglePlayback.
• Removes obsolete Message::MockAction.

Verification

• ./scripts/test.sh passed cleanly (fmt, clippy, tests, deny, audit, typos, lychee, markdownlint, release build, docs).