Upgrade build pipeline to Vite 8

[Copilot]

This updates the frontend build toolchain to Vite 8 and applies the required migration away from deprecated Rollup-specific config paths. It also removes Vite 7-only integration code and dependencies that are no longer compatible/needed.

• Dependency upgrades and removals

  • Upgraded vite from ^7.3.1 to ^8.0.11.
  • Upgraded @preact/preset-vite to ^2.10.5 to match Vite 8 peer support.
  • Removed @kucrut/vite-for-wp (locked to Vite 7 peer range).

• Vite 8 migration

  • Migrated build config from build.rollupOptions to build.rolldownOptions in:
    • vite.config.js
    • vite.extension.config.js
  • Kept output contract unchanged (query-monitor.js, stable asset names, existing out dirs).

• Replacement of removed plugin behavior

  • Replaced v4wp() usage with explicit native Vite config (base, build.outDir, emptyOutDir, modulePreload, css.devSourcemap, dev server settings).
  • Added a small local Vite plugin to emit/remove assets/build/vite-dev-server.json during dev so existing PHP manifest detection continues to work.

build: {
  rolldownOptions: {
    input: {
      main: 'src/index.tsx',
      'query-monitor': 'assets/query-monitor.css',
      toolbar: 'assets/toolbar.css',
    },
    output: {
      entryFileNames: 'query-monitor.js',
      assetFileNames: '[name][extname]',
    },
  },
}