style: rework user message styling from violet to cyan theme

Replace violet tint with cyan/teal tones to match the existing cyan accent
theme. User messages now use from-cyan-900/20 background gradient and
cyan-400/15 border, with cyan-200 user icon. Subtler and more cohesive
with the dark theme while remaining clearly distinct from assistant messages.

Fixes feedback #44

src/components/ChatMessage.tsx

@@ -293,7 +293,7 @@ export function ChatMessageComponent({ message, onRetry }: { message: ChatMessag
       {/* Avatar */}
       <div className="shrink-0 mt-1 flex h-9 w-9 items-center justify-center rounded-2xl border border-white/8 bg-zinc-800/40">
         {isUser
-          ? <User className="h-4 w-4 text-violet-200" />
+          ? <User className="h-4 w-4 text-cyan-200" />
           : <Bot className="h-4 w-4 text-cyan-200" />
         }
       </div>
@@ -302,7 +302,7 @@ export function ChatMessageComponent({ message, onRetry }: { message: ChatMessag
       <div className={`min-w-0 max-w-[80%] ${isUser ? 'text-right' : ''}`}>
         <div className={`group relative inline-block text-left rounded-3xl px-4 py-3 text-sm leading-relaxed max-w-full overflow-hidden ${
           isUser
-            ? 'bg-gradient-to-b from-violet-900/20 to-zinc-900/60 text-zinc-200 border border-violet-400/15'
+            ? 'bg-gradient-to-b from-cyan-900/20 to-zinc-900/60 text-zinc-200 border border-cyan-400/15'
             : 'bg-zinc-800/40 text-zinc-300 border border-white/8 shadow-[0_0_0_1px_rgba(255,255,255,0.03)]'
         }`}>
           {/* Copy button (assistant messages only) */}