Add artist-based library grouping and configurable ordering (alphabet/age/random)

[Copilot]

This change extends library filtering/sorting so users can browse by artist as well as album, and switch group ordering between alphabetic, age-based, and random. It also changes album song listing from title-based sorting to track-order-first behavior.

• Library model: grouping + ordering modes

  • Added Library modes:
    • group_mode: album | artist
    • order_mode: alphabet | age | random
  • Added mode setters and centralized rebuild logic so rendering/search stay consistent across mode changes.

• Metadata-aware ordering

  • Added metadata extraction for:
    • artist (TPE1, artist, etc.)
    • track number (TRCK, tracknumber, etc.)
    • year/date (TDRC, TYER, etc.)
  • Album groups now render songs by track number (fallback to scan/index order), not by alphabetical title.
  • Age ordering sorts groups by earliest available year; unknown-year groups are placed after known-year groups.

• Filtering behavior

  • Search now matches against song title + album + artist.
  • Group headers are included when either the header or any contained song matches.

• UI controls and layout

  • Added compact top-bar controls in Library view:
    • By album / By artist
    • A->Z / By age / Random
  • Reflowed top-row rects so new controls and search box do not overlap existing elements.

• Docs/tests updates

  • Updated README feature bullets for artist filtering/grouping and order modes.
  • Added focused unit tests for:
    • grouping by artist + artist-name filtering
    • age-based group ordering
  • Updated existing render expectations to include enriched song metadata fields used by search/grouping.

# New Library API surface used by UI:
lib.set_group_mode("artist")      # or "album"
lib.set_order_mode("age")         # "alphabet" | "age" | "random"
lib.set_search("artist b")        # matches artist/album/song