fix: Darker modal backdrops for better contrast

SettingsModal + ArtworkGenerationModal:
- Backdrop: bg-black/60 → bg-black/80 (darker, less bleed-through)
- Backdrop blur: backdrop-blur-sm → backdrop-blur-md (stronger blur effect)
- Modal surface: added lyria-surface color (#1a1a24) to tailwind config
  for solid, opaque modal backgrounds

Result: Background elements are much less visible behind modals,
improving readability and focus.

frontend/tailwind.config.js

@@ -9,6 +9,7 @@ export default {
       colors: {
         'lyria-dark': '#0a0a0f',
         'lyria-panel': '#12121a',
+        'lyria-surface': '#1a1a24',
         'lyria-accent': '#ff6b35',
         'lyria-text': '#e0e0e0',
         'lyria-muted': '#6b6b7b',