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/css/style.css

@@ -195,6 +195,18 @@ body.resizing iframe { pointer-events: none; }
 }
 .auto-run-toggle input { cursor: pointer; }
 
+/* Tailwind toggle */
+.tailwind-toggle {
+  display: flex;
+  align-items: center;
+  gap: 4px;
+  font-size: 12px;
+  color: var(--text-dim);
+  cursor: pointer;
+  user-select: none;
+}
+.tailwind-toggle input { cursor: pointer; }
+
 /* Dividers */
 .divider { background: var(--border); transition: background 0.15s; z-index: 2; }