[good first issue] Add File Upload example page + tests

[kevinccbsg]

Real-world pattern not currently covered (<input type="file">).

Scope

• New page examples/twd-test-app/src/pages/FileUpload.tsx:
  • A single file input.
  • Display the selected filename.
  • Reject non-image files with a visible error message.
• Register the route in examples/twd-test-app/src/routes.tsx and add a link from the App menu.
• New test file examples/twd-test-app/src/twd-tests/file-upload.twd.test.ts:
  • Query the input with screenDom.getByLabelText(...).
  • Happy path: userEvent.upload(input, file) → filename appears (screenDom.getByText(...) + twd.should(el, "be.visible")).
  • Invalid file type: error appears (screenDom.getByRole("alert") or screenDom.getByTestId("upload-error")).

Reference

• Page structure: examples/twd-test-app/src/pages/ComboboxSelect.tsx.
• Test structure: examples/twd-test-app/src/twd-tests/combobox-select.twd.test.ts.
• Query pattern: examples/twd-test-app/src/twd-tests/screen-queries.twd.test.ts.

Acceptance criteria

• New page renders and is reachable from the App menu.
• Two test cases run green in the TWD sidebar.
• PR includes a sidebar screenshot and a screenshot of the new page.

How to contribute

Read CONTRIBUTING.md first. Quick steps for this issue:

1. Fork, clone, create a feature branch: git checkout -b feat/file-upload-example.
2. From the repo root, build the library and sync it to the example apps:

   npm install
   npm run build
   npm run copy:mock-sw

3. Install and run the test app:

   cd examples/twd-test-app
   npm install
   npm run dev

4. Open the app in the browser — the TWD sidebar appears on the right. Run your new tests from the sidebar and confirm they pass.
5. Run the library's unit tests from the repo root: npm run test:ci.
6. Open the PR with screenshots of the sidebar showing green tests.

Always prefer screenDom (Testing Library) for element queries. Avoid twd.get() in new code.