Add React Testing Library and PhotoViewer component/hook tests

[kulcsarrudolf]

Problem

The test suite currently consists of two pure utility test files (src/utils/photo-viewer/__tests__/transformUtils.test.ts and gestureUtils.test.ts).
There are zero component or hook tests, React Testing Library is not installed, and Jest runs with --passWithNoTests.
PhotoViewer was recently refactored into 9 hooks and several sub-components (#7, PR #13), which makes it very testable, but nothing locks in its behavior.

Proposed change

1. Add dev dependencies: @testing-library/react, @testing-library/user-event, @testing-library/jest-dom.
   These are dev-only, so the zero-runtime-dependency guarantee is unaffected.
2. Add a jest setup file wiring in @testing-library/jest-dom.
3. Write PhotoViewer tests covering at least:
   • Opens with the expected image and closes via the close control and the Escape key.
   • Next/previous navigation updates the displayed image, including wrap-around behavior.
   • Keyboard shortcuts from useKeyboardShortcuts (arrow keys, Escape, zoom keys if any).
   • Zoom interaction updates the transform (integration with useTransform / useWheelZoom).
   • Image preloading behavior from useImagePreloader (adjacent images requested).

MediaGrid and ImageCarousel tests are tracked as separate follow-up issues so this one stays reviewable.

Acceptance criteria

• RTL infrastructure in place and documented in CONTRIBUTING.md.
• PhotoViewer interaction tests passing in CI.
• --passWithNoTests removed from the test script.

Compatibility

Dev dependencies and tests only.
No runtime or API changes.

[kulcsarrudolf]

Partially implemented by #21. Leaving this open, since the infrastructure landed but most of the requested coverage did not.

Done:

• @testing-library/react, @testing-library/jest-dom, and @testing-library/dom are in devDependencies
• jest setup wiring @testing-library/jest-dom is in place
• src/components/photo-viewer/__tests__/PhotoViewer.test.tsx exists

Still outstanding. The existing suite has three tests (renders nothing when no image is selected, renders the selected image, closes on Escape). Of the five areas listed above, only "opens with the expected image / closes via Escape" is covered. Missing:

• closing via the close control (as opposed to the Escape key)
• next/previous navigation updating the displayed image, including wrap-around
• the remaining useKeyboardShortcuts bindings (ArrowLeft/ArrowRight, +/=/-, r, 0)
• zoom interaction updating the transform (useTransform / useWheelZoom)
• useImagePreloader requesting adjacent images

Also still open from the acceptance criteria:

• --passWithNoTests is still present in the test script
• RTL is not yet documented in CONTRIBUTING.md

Note that @testing-library/user-event is not installed yet; it will be needed for the interaction tests.