Fixes a React runtime error where `useRef` was null due to multiple React instances being bundled.
## Changes Made
- **Vite configuration**: Added `resolve.dedupe` for `react` and `react-dom` in `examples/mcp/vite.config.ts` to ensure only one instance of React is used during bundling
- **React version constraints**: Changed from exact versions (`19.2.3`) to flexible ranges (`^19.0.0`) in `examples/mcp/package.json` to improve compatibility
- **Build entry point**: Added new `build-app-html-entry.ts` file and updated `tsup.config.ts` to include it as a build target
- **Package exports**: Added `./build-app-html` export path in `packages/mcp/package.json`
- **Minor fixes**: Updated Next.js type reference path in stripe-app example
The root cause was React being duplicated in the bundle, causing hooks like `useRef` to fail when components tried to access React's internal state from the wrong instance. The `dedupe` configuration ensures Vite resolves all React imports to the same module instance.
Fixes#190
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