chore: upgrade plugins to next 16

[jhb-dev]

Summary

• Bumps every plugin + dev app to Next.js 16 and broadens the peer dep range to ^15.0.0 || ^16.0.0.
• Migrates each dev app's next.config.mjs → next.config.ts and removes unused nodemon.json / plugin.spec.ts leftovers.
• Switches dev app tsconfig.json to module: NodeNext / moduleResolution: NodeNext, which is required for Turbopack (the Next 16 default) to resolve .js ESM specifiers back to .ts/.tsx sources. With Bundler resolution, Turbopack fails on imports like export { SearchBar } from '../components/SearchBar/SearchBar.js' (tracked upstream in Turbopack: support importing .ts/.tsx via .js extension (parity with webpack resolve.extensionAlias) vercel/next.js#82945).
• Picks up Next 16's mandatory jsx: preserve → jsx: react-jsx and new .next/dev/types tsconfig additions.
• Restructures all dev apps to match the official Payload plugin template pattern: removes dev app package.json files and pnpm workspace entries, moves dev dependencies to the plugin root devDependencies, rewrites dev/tsconfig.json to extend the root config with plugin alias paths, and adds .js extensions to all relative imports for NodeNext resolution. This eliminates duplicate @payloadcms/ui instances that caused useConfig() / useField() context errors at runtime.

Test plan

• pnpm test:all — passes across all plugins (chat-agent 269, pages localized 64×2, unlocalized 12×2, multi-tenant 7×2, vercel-deployments 23, plus admin-search/alt-text/astro/content-translator Node strip-types suites)
• pnpm lint:all — 0 errors (warnings only, pre-existing)
• pnpm format:all — clean
• Started every dev app under Turbopack and curled /admin: admin-search, alt-text/dev, alt-text/dev_unlocalized, chat-agent, cloudinary, content-translator, geocoding, pages/dev, pages/dev_multi_tenant, pages/dev_unlocalized, vercel-deployments → all HTTP 200