fix: use theme CSS variables for user messages

User messages were using hard-coded indigo colors instead of respecting
the selected theme. Now properly uses:
- var(--accent) for gradient start
- var(--user-msg) for gradient end
- Dynamic box-shadow based on theme color

This ensures user messages match the selected theme (Dark, Light, Abyss,
Nord, Mocha, etc.)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/styles.css

@@ -276,10 +276,10 @@ body {
 }
 
 .message.user .message-content {
-  background: linear-gradient(135deg, #6366f1 0%, #4f46e5 100%);
+  background: linear-gradient(135deg, var(--accent) 0%, var(--user-msg) 100%);
   color: white;
   border-bottom-right-radius: 4px;
-  box-shadow: 0 2px 8px rgba(99, 102, 241, 0.3);
+  box-shadow: 0 2px 8px color-mix(in srgb, var(--user-msg) 30%, transparent);
 }
 
 .message.assistant .message-content {