fix(deps): replace webpack with esbuild for browser bundling

Eliminates serialize-javascript RCE vulnerability (transitive via
webpack → terser-webpack-plugin). IIFE format with globalName is
functionally equivalent to UMD for script-tag usage.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: vveerrgg

dist/browser/nostr-websocket-utils.min.js

@@ -0,0 +1,20 @@
+import { build } from 'esbuild';
+
+const result = await build({
+  entryPoints: ['src/browser.ts'],
+  bundle: true,
+  minify: true,
+  sourcemap: true,
+  format: 'iife',
+  globalName: 'NostrWebSocketUtils',
+  outfile: 'dist/browser/nostr-websocket-utils.min.js',
+  target: ['es2020'],
+  platform: 'browser',
+  metafile: true,
+});
+
+// Print bundle size info
+const output = Object.entries(result.metafile.outputs)
+  .filter(([k]) => k.endsWith('.js'))
+  .map(([k, v]) => `${k}: ${(v.bytes / 1024).toFixed(1)}KB`);
+console.log('Browser bundle built:', output.join(', '));