Add Tailwind CSS toggle, Markdown/WASM modes, and npm import resolution

- Tailwind CSS: toolbar checkbox injects Play CDN into preview, persisted
  per-fiddle via new options JSON column
- Markdown mode: uses marked.js CDN, renders markdown to HTML preview with
  CSS tab for custom styling
- WASM mode: starter template with inline WebAssembly add function, supports
  top-level await via module detection
- npm imports: auto-detect bare import specifiers in module code and inject
  importmap pointing to esm.sh CDN
- Module auto-detection for html-css-js mode (import/export statements)
- DB migration adds options column, server passes through all API endpoints
- All features work across preview, export, and embed

public/js/import-map.js

@@ -0,0 +1,43 @@
+/**
+ * Extract bare import specifiers from JS code.
+ * Matches `from 'pkg'`, `from "pkg"`, `import 'pkg'`, `import "pkg"`
+ * where the specifier doesn't start with `.`, `/`, `http:`, or `https:`.
+ */
+export function extractBareImports(jsCode) {
+  const imports = new Set();
+  const re = /(?:import\s+(?:[\s\S]*?\s+from\s+)?['"])([^'"\s][^'"]*?)(?=['"])/g;
+  let match;
+  while ((match = re.exec(jsCode)) !== null) {
+    const spec = match[1];
+    // Skip relative, absolute, and URL imports
+    if (spec.startsWith('.') || spec.startsWith('/') || spec.startsWith('http:') || spec.startsWith('https:')) {
+      continue;
+    }
+    // Skip svelte internal imports (already rewritten by the compiler)
+    if (spec.startsWith('svelte')) continue;
+    imports.add(spec);
+  }
+  return Array.from(imports);
+}
+
+/**
+ * Build an HTML <script type="importmap"> tag from bare import specifiers.
+ * Maps each bare import to its esm.sh URL.
+ */
+export function buildImportMapTag(bareImports) {
+  if (!bareImports.length) return '';
+  const imports = {};
+  for (const spec of bareImports) {
+    // Get the package name (handle scoped packages like @scope/pkg)
+    const parts = spec.split('/');
+    const pkgName = spec.startsWith('@') ? `${parts[0]}/${parts[1]}` : parts[0];
+    // Map the full specifier (e.g. 'lodash/fp' -> 'https://esm.sh/lodash/fp')
+    imports[spec] = `https://esm.sh/${spec}`;
+    // Also map the base package if it's different
+    if (spec !== pkgName) {
+      imports[pkgName] = `https://esm.sh/${pkgName}`;
+    }
+  }
+  const map = JSON.stringify({ imports }, null, 2);
+  return `<script type="importmap">\n${map}\n<\/script>`;
+}