Export curated PhotoViewer hooks as public API

[kulcsarrudolf]

Motivation

The PhotoViewer refactor (#7, PR #13) produced a set of well-scoped hooks in src/hooks/photo-viewer/, but none are exported.
Consumers who want a custom viewer UI with the same zoom/gesture behavior currently have to reimplement them.

Proposed change

Export a curated subset from src/index.ts, the ones that are generally reusable:

• useTransform
• useWheelZoom
• useTouchGestures
• useKeyboardShortcuts

Deliberately keep the viewer-internal glue private (useImageNavigation, useImagePreloader, useMouseDrag, useClickOutsideToClose, usePhotoViewerSettings) to preserve refactoring freedom; they can graduate later if there is demand.

Each exported hook gets:

• TSDoc comments on the hook and its options/return types.
• A "Hooks" section in the README with a minimal custom-viewer example.
• Type exports for their option/return shapes.

Best done after component/hook tests exist, so the exported behavior is locked in before it becomes public API.

Acceptance criteria

• The four hooks importable from the package root with full types.
• README documents each with an example.
• Tests cover the exported hook contracts.

Compatibility

Purely additive API.
Existing exports unchanged.