add WebGL scenes to all themes, sounds, image sizing, new themes

- All 11 themes now have unique Three.js WebGL backgrounds:
  aurora: flowing sine-wave ribbon meshes with additive blending
  brutalist: harsh rotating white/red wireframe boxes
  glass: drifting crystal icosahedron shards, semi-transparent
  matrix: digital rain particle columns with per-vertex colour fade
  ocean: animated vertex-displaced wave surface with orbiting light
  retro: amber demo-scene cube with orbiting octahedrons
  synthwave: sunset sphere with scan-line rings and perspective grid
  terminal: green icosahedron wireframe with orbiting torus particles
  neon: existing Three.js orb and rings (unchanged)
  plasma (replaces minimal): drifting additive-blend colour blobs
  volcano (replaces paper): rising ember particles with lifecycle

- shared.go: distinct sounds for j/k nav (220Hz beep), Enter (ascending
  triangle chime), and Esc (descending sine sweep)
- shared.go: images are now thumbnails (max-height:220px) in list view
  and expand to full width inside the modal (CSS override in navmodal)
- main.go: --list-themes flag prints all theme names and exits;
  --theme random picks a random theme at all generation time
- themes.go: updated registry with plasma/volcano replacing minimal/paper

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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