feat: add Create Playlist feature

[SkyeVault]

What this adds

Adds a Create Playlist feature to the Playlists panel. A + Add Playlist
entry appears at the bottom of the playlist list. Pressing Enter opens a
two-stage form:

Stage 1 — Name your playlist

• Type the playlist name and press Enter to continue

Stage 2 — Add tracks

• Search for tracks by name/artist
• Left panel shows search results, right panel shows tracks added so far
• Tab switches focus between search input and results
• Enter adds the highlighted track to your playlist
• d removes a track from the added list
• Enter while in the added tracks panel creates the playlist on Spotify

Bug fixes included

• Silences 404 from me/player on startup when no device is active (treated as idle instead of showing an error)
• Fixes crash from duplicate items key in Spotify API JSON responses on me/playlists and playlist creation endpoints

Testing

• cargo build passes clean
• All 145 existing tests pass (cargo test)
• Manually tested: playlist creation with and without tracks, cancellation, error states

[LargeModGames]

@all-contributors add @SkyeVault for code

[allcontributors]

@LargeModGames

I've put up a pull request to add @SkyeVault! 🎉