Add album art display options to music live activity

[lasersushi]

Summary

Adds a new "Album art display" picker in Media settings with three options:

• Always show (default) — current behavior, no change for existing users
• Fade after 3 seconds — album art appears briefly then gradually fades out for a more minimal look; re-appears on song change, hover, or when the notch reopens
• Show app icon — replaces the album art with the icon of the currently-playing app (Spotify, Apple Music, etc.)

Implements two Discord feature requests:
https://discord.com/channels/1269588937320566815/1493582011678330962

Changes

• Constants — new AlbumArtDisplayMode enum (.always, .fade, .appIcon) + albumArtDisplayMode Defaults key, default .always
• MediaSettingsView — new Album art display picker, placed under Media inactivity timeout
• ContentView — MusicLiveActivity() now switches image source and applies a cancellable fade timer based on the selected mode, reacting to song changes, notch state, and hover via .onAppear + .onChange
• Localizable.xcstrings — new English keys for the picker label and three options

Testing Done in Xcode

• Always show — verified album art renders identically to before; no regression
• Fade after 3s:
  • Art fades out ~3s after notch settles closed
  • Song change resets the fade; art reappears at full opacity then fades again
  • Hovering cancels the fade and snaps to full opacity; moving away restarts the countdown
  • Opening the notch cancels the fade; closing it restarts the countdown
• Show app icon — app icon correctly replaces album art; falls back to album art if no bundle ID is available yet

Screenshots/Screen Recordings

Screen.Recording.2026-05-11.at.12.51.57.PM.mov