feat(llm-playground): add SvelteKit LLM playground UI

- Chat interface with message history
- Model selector for available LLM models
- Parameter panel (temperature, max tokens, etc.)
- System prompt editor
- Svelte 5 runes-based stores

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

services/llm-playground/src/app.css

@@ -0,0 +1,103 @@
+@import 'tailwindcss';
+
+:root {
+	--color-bg: #0f0f0f;
+	--color-surface: #1a1a1a;
+	--color-surface-hover: #252525;
+	--color-border: #333;
+	--color-text: #e5e5e5;
+	--color-text-muted: #888;
+	--color-primary: #3b82f6;
+	--color-primary-hover: #2563eb;
+	--color-success: #22c55e;
+	--color-error: #ef4444;
+	--color-warning: #f59e0b;
+}
+
+html {
+	background-color: var(--color-bg);
+	color: var(--color-text);
+}
+
+body {
+	font-family:
+		'Inter',
+		-apple-system,
+		BlinkMacSystemFont,
+		'Segoe UI',
+		Roboto,
+		sans-serif;
+}
+
+/* Custom scrollbar */
+::-webkit-scrollbar {
+	width: 8px;
+	height: 8px;
+}
+
+::-webkit-scrollbar-track {
+	background: var(--color-surface);
+}
+
+::-webkit-scrollbar-thumb {
+	background: var(--color-border);
+	border-radius: 4px;
+}
+
+::-webkit-scrollbar-thumb:hover {
+	background: #444;
+}
+
+/* Prose styles for markdown */
+.prose {
+	line-height: 1.6;
+}
+
+.prose pre {
+	background: var(--color-bg);
+	padding: 1rem;
+	border-radius: 0.5rem;
+	overflow-x: auto;
+	margin: 0.5rem 0;
+}
+
+.prose code {
+	background: var(--color-bg);
+	padding: 0.125rem 0.375rem;
+	border-radius: 0.25rem;
+	font-size: 0.875em;
+}
+
+.prose pre code {
+	background: transparent;
+	padding: 0;
+}
+
+.prose p {
+	margin: 0.5rem 0;
+}
+
+.prose ul,
+.prose ol {
+	margin: 0.5rem 0;
+	padding-left: 1.5rem;
+}
+
+.prose li {
+	margin: 0.25rem 0;
+}
+
+.prose h1,
+.prose h2,
+.prose h3 {
+	margin-top: 1rem;
+	margin-bottom: 0.5rem;
+	font-weight: 600;
+}
+
+.prose blockquote {
+	border-left: 3px solid var(--color-border);
+	padding-left: 1rem;
+	margin: 0.5rem 0;
+	color: var(--color-text-muted);
+}