build: adopt esbuild for frontend bundling (build:web → public/build/)

- esbuild dev dep; build:web/dev:web bundle public/main.ts → public/build/main.js
- gitignore public/build/; tsconfig.web.json now typecheck-only
- frontend build convention documented in PLAN §1, DISPATCH, ARCHITECTURE §5
- resolves the T1 deferred decision

package.json

@@ -12,7 +12,8 @@
     "start": "tsx src/server.ts",
     "dev": "tsx watch src/server.ts",
     "build": "tsc -p tsconfig.json",
-    "build:web": "tsc -p tsconfig.web.json",
+    "build:web": "esbuild public/main.ts --bundle --format=esm --outdir=public/build --sourcemap",
+    "dev:web": "esbuild public/main.ts --bundle --format=esm --outdir=public/build --sourcemap --watch",
     "typecheck": "tsc -p tsconfig.json --noEmit && tsc -p tsconfig.web.json",
     "test": "vitest run",
     "test:watch": "vitest"
@@ -28,6 +29,7 @@
     "@types/express": "^5.0.6",
     "@types/node": "^25.9.3",
     "@types/ws": "^8.18.1",
+    "esbuild": "^0.28.1",
     "tsx": "^4.22.4",
     "typescript": "^6.0.3",
     "vitest": "^4.1.9"