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-lock.json

@@ -7,6 +7,7 @@
     "": {
       "name": "web-terminal",
       "version": "0.1.0",
+      "hasInstallScript": true,
       "dependencies": {
         "@xterm/addon-fit": "^0.11.0",
         "@xterm/xterm": "^6.0.0",
@@ -18,6 +19,7 @@
         "@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"