feat(hub): draw an actual CD for covers with no art, not a flat icon

The monochrome disc icon from the previous commit read as placeholder
UI chrome, not as artwork - too plain for something that is, for most
tiles in a real library, the default look of the grid.

- DiscPlaceholder (music-app.js): a small inline SVG illustration -
  dark disc base, an iridescent radial-gradient sheen mimicking the
  rainbow reflection a real CD's data side has, two faint groove
  rings, a light label ring, a dark spindle hole. Each instance gets
  its own gradient id (a module-level counter) rather than one literal
  id repeated - a grid renders many of these at once, and a shared id
  would leave every disc after the first pointing at whichever
  <radialGradient> the browser happened to resolve.
- AlbumCard and MusicDetailModal now branch on coverHash directly:
  MediaThumb (the real chunk-path image) when there is one,
  DiscPlaceholder when there isn't - rather than routing "no cover"
  through MediaThumb's own generic small-icon fallback, which is still
  right for its other callers (list rows, Videos).
- icon.js: the flat monochrome "disc" icon this replaces is removed -
  nothing else used it.
- style.css: .music-disc-empty/.music-disc-svg replace the old
  .video-thumb-empty overrides; still sized relative to the tile (82%)
  so it scales with .music-grid's auto-fill columns.

Client-side only. npm run sync-ui re-run. Full suite: 1129 passed, no
regressions.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KBi7ALLGfwcjBXt57yNMcy

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