Clean codegen leftovers from mizan-react after the protocol/ relocation

The codegen used to live in mizan-react before mizan-django before
protocol/mizan-generate. Each move left sediment in the previous
home; the bin entry in particular shadowed mizan-generate's own bin
in node_modules/.bin/, breaking `npx mizan-generate`. Caught at
integration time when the harness install picked up the stale link.

frontends/mizan-react/package.json:
- Removed bin entry pointing at the long-gone ./dist/generator/cli.mjs.
- Simplified the build script — dropped `cpSync('src/generator',
  'dist/generator', ...)`. src/generator hasn't existed in this package
  since the first move; the cpSync would silently fail at every build.
- Removed optionalDependencies (chokidar, minimatch, openapi-typescript) —
  these were codegen-watcher deps, no longer relevant to the React adapter.

examples/{django,fastapi}-react-site/harness/package.json:
- Added `mizan-generate` as a file: devDep so `npx mizan-generate
  --config <config.mjs>` resolves to the right binary in the monorepo.
  Mirrors the install pattern the README documents for downstream users.

Verified: mizan-react vitest 33/33 (78 skipped — integration tests).
Codegen runs from harness via npx for both example apps.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

examples/django-react-site/harness/package.json

@@ -17,6 +17,7 @@
     "@types/react": "^19.0.0",
     "@types/react-dom": "^19.0.0",
     "@vitejs/plugin-react": "^4.0.0",
+    "mizan-generate": "file:../../../protocol/mizan-generate",
     "typescript": "^5.7.0",
     "vite": "^6.0.0"
   }