⚡ refactor: Migrate @librechat/client build to tsdown (#13596)

* ⚡ refactor: Migrate @librechat/client build from Rollup to tsdown

Mirrors the data-schemas migration. Replaces Rollup (rpt2 + postcss) with
tsdown (rolldown + oxc); the package build drops from tens of seconds to ~0.3s.

- Emit isolated-declaration .d.ts via oxc (dts.oxc) and enforce
  isolatedDeclarations in tsconfig for editor DX (source made clean: explicit
  export type annotations added across src, no `any`).
- Extract component CSS to dist/style.css so the CJS output stays valid
  CommonJS (the prior postcss runtime-injection produced an ESM import in the
  CJS bundle that breaks jest/require). Imported once in the client app entry;
  Vite bundles it for the app.
- Repoint package.json to dual .mjs/.cjs + .d.mts/.d.cts and add ./style.css
  and ./package.json exports.
- Update CI build-cache keys to hash tsdown.config.mjs; remove rollup.config.js.

* 🔧 chore: address Codex review on client tsdown migration

- Add tsdown.config.mjs to turbo.json build `inputs` so changes to the new
  bundler config invalidate the Turbo cache (the shared inputs only listed the
  rollup configs). Also covers the already-migrated data-schemas.
- Name the memoized default export (ControlComboboxMemo) instead of the
  codefix-generated `_default_1`, for clearer stack traces / grepping.

client/src/main.jsx

@@ -2,6 +2,7 @@ import './polyfills/regeneratorRuntime';
 import { createRoot } from 'react-dom/client';
 import './locales/i18n';
 import App from './App';
+import '@librechat/client/style.css';
 import './style.css';
 import './mobile.css';
 import { ApiErrorBoundaryProvider } from './hooks/ApiErrorBoundaryContext';