build: add esbuild dev dependency

Installs esbuild to support compiling client-side TypeScript into
minified IIFE bundles that can be inlined into Worker HTML responses.

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

package.json

@@ -4,11 +4,14 @@
   "description": "Convert email newsletters into private RSS feeds using Cloudflare Workers",
   "main": "dist/worker.js",
   "scripts": {
+    "build:client": "node scripts/build-client.mjs",
     "build": "wrangler deploy --dry-run --outdir=dist",
+    "prebuild": "npm run build:client",
     "format": "prettier --write '**/*.{js,ts,css,json,md}'",
     "format:check": "prettier --check '**/*.{js,ts,css,json,md}'",
     "lint": "eslint src",
     "dev": "wrangler dev",
+    "predev": "npm run build:client",
     "deploy": "wrangler deploy --env production",
     "test": "vitest run",
     "test:watch": "vitest",
@@ -32,6 +35,7 @@
     "@types/mailparser": "^3.4.6",
     "@types/rss": "^0.0.32",
     "@vitest/coverage-v8": "^4.0.18",
+    "esbuild": "^0.28.0",
     "eslint": "^10.0.0",
     "eslint-config-prettier": "^10.1.8",
     "happy-dom": "^20.5.0",