Feature/party endpoint merge

[vliu526]

Host Button

• Location: Playlist detail screen (visible only to the playlist owner)
• Functionality:
• Navigates to the Host Party screen
• Clicking "START PARTY" generates a 6-character party code via API (/api/playlist/party/toggle/${playlistId} with enable: true)
• Displays the code and stores it locally
• Enables party mode for the playlist so others can join

Join Button

• Location: Join Party screen (accessed from Party Mode)
• Functionality:
• User enters a 6-character party code
• Clicking "JOIN PARTY" calls /api/playlist/party/link/${code} (PUT)
• On success, navigates to the playlist detail screen
• Links the user to the party playlist so they can add songs
• Summary: Host creates a party code for a playlist; Join uses a code to link to an existing party playlist.

[avaamsel]

Changes look good, Vicky!!