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

db.js

@@ -31,6 +31,11 @@ try {
   db.exec(`ALTER TABLE fiddles ADD COLUMN listed INTEGER NOT NULL DEFAULT 1`);
 } catch (_) { /* column already exists */ }
 
+// Migration: add options column (JSON string for per-fiddle settings like tailwind)
+try {
+  db.exec(`ALTER TABLE fiddles ADD COLUMN options TEXT NOT NULL DEFAULT '{}'`);
+} catch (_) { /* column already exists */ }
+
 // Tags tables
 db.exec(`
   CREATE TABLE IF NOT EXISTS tags (
@@ -51,14 +56,14 @@ db.exec(`CREATE INDEX IF NOT EXISTS idx_fiddles_listed_updated ON fiddles(listed
 
 export const stmts = {
   insert: db.prepare(`
-    INSERT INTO fiddles (id, title, html, css, css_type, js, js_type, listed)
-    VALUES (@id, @title, @html, @css, @css_type, @js, @js_type, @listed)
+    INSERT INTO fiddles (id, title, html, css, css_type, js, js_type, listed, options)
+    VALUES (@id, @title, @html, @css, @css_type, @js, @js_type, @listed, @options)
   `),
   get: db.prepare('SELECT * FROM fiddles WHERE id = ?'),
   update: db.prepare(`
     UPDATE fiddles SET title = @title, html = @html, css = @css,
       css_type = @css_type, js = @js, js_type = @js_type, listed = @listed,
-      updated_at = datetime('now')
+      options = @options, updated_at = datetime('now')
     WHERE id = @id
   `),
   list: db.prepare('SELECT id, title, css_type, js_type, created_at, updated_at FROM fiddles ORDER BY updated_at DESC LIMIT 50'),