Refactor updatePlaylistImage function and comments

[rosgr100]

Refactor playlist image update function to use playlist ID instead of URI. Update various comments and improve HTML entity decoding.

fix(savePlaylists): replace broken sp:// endpoints with Platform API

Fix: Replace broken sp:// and Web API endpoints with Platform API

Problem

The extension is completely broken on current Spotify/Spicetify versions. All three sp://core-playlist Cosmos endpoints have been removed and return Resolver not found!. Direct calls to api.spotify.com consistently return 429 Too Many Requests when made from within the Spotify client.

Errors seen:

• GET sp://core-playlist/v1/playlist/... Resolver not found!
• POST sp://core-playlist/v1/rootlist ... Resolver not found!
• PUT https://api.spotify.com/v1/playlists/... 429 Too Many Requests

Fix

Replaced all broken calls with Spicetify.Platform API equivalents, which route through Spotify's internal transport and are not rate-limited:

Before	After
CosmosAsync.get(sp://core-playlist/v1/playlist/)	Platform.PlaylistAPI.getMetadata() + getContents()
CosmosAsync.post(sp://core-playlist/v1/rootlist)	Platform.RootlistAPI.createPlaylist()
CosmosAsync.post(api.spotify.com/.../tracks)	Platform.PlaylistAPI.add() in chunks of 100
fetch PUT api.spotify.com/.../playlists (description)	Platform.PlaylistAPI.setAttributes()

Also fixed: HTML entities in descriptions (e.g. /) are now decoded before writing, and tracks are paginated correctly for playlists over 100 tracks.

Legacy sp:// fallbacks are kept for older Spicetify versions where Platform.PlaylistAPI may not be available.

Fixes #264