feat: festive Chinese red background and prize config unification

- Change all big screen backgrounds to pure Chinese red (#c41230)
- Remove vignette effect for consistent display on different screens
- Unify all prize data to match prizes.json configuration
- Add demo participants fallback when no data imported
- Fix particle visibility reset in startGalaxy()

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/client-screen/src/assets/styles/global.scss

@@ -10,7 +10,8 @@ html, body {
   width: 100%;
   height: 100%;
   overflow: hidden;
-  background: $color-bg-dark;
+  // Chinese New Year festive pure red background
+  background: #c41230;
   font-family: 'PingFang SC', 'Microsoft YaHei', sans-serif;
   color: $color-text-light;
   -webkit-font-smoothing: antialiased;

packages/client-screen/src/assets/styles/variables.scss

@@ -11,10 +11,10 @@ $color-gold-light: #ffd966;
 $color-gold-dark: #d4a84b;
 $color-gold-deep: #b8923f;
 
-// Cinematic background
-$color-bg-dark: #0a0a0a;
-$color-bg-wine: #2a0a0e;
-$color-bg-gradient: linear-gradient(180deg, #2a0a0e 0%, #1a0808 50%, #0a0505 100%);
+// Cinematic background - Festive Chinese Red
+$color-bg-dark: #c41230;
+$color-bg-wine: #c41230;
+$color-bg-gradient: linear-gradient(180deg, #d4232a 0%, #c41230 50%, #a01028 100%);
 
 // Text
 $color-text-light: #ffffff;