feat(themes): add synthwave theme with neon pink palette

Adds a synthwave preset — deep indigo background (#0d0221), neon pink
accent glow (#ff2d78), and crisp off-white text (#f8f8f2). Includes
THEMES.md entry and preview screenshot.

Author: Priyanshu-byte-coder

THEMES.md

@@ -81,3 +81,11 @@ Usage: `/api/streak?user=yourusername&theme=rose`
 ![Nord](assets/themes/nord.png)
 
 Usage: `/api/streak?user=yourusername&theme=nord`
+
+## 🌆 Synthwave
+
+![Synthwave](assets/themes/synthwave.png)
+
+Retro 80s-inspired palette — deep indigo background, neon pink glow, crisp white text.
+
+Usage: `/api/streak?user=yourusername&theme=synthwave`

assets/themes/synthwave.png

@@ -52,4 +52,9 @@ export const themes: Record<string, BadgeTheme> = {
     text: 'd8dee9',
     accent: '88c0d0',
   },
+  synthwave: {
+    bg: '0d0221',
+    text: 'f8f8f2',
+    accent: 'ff2d78',
+  },
 };