deps(deps-dev): bump vitest and @vitest/coverage-v8 to v4 (#257)

* deps(deps-dev): bump vitest and @vitest/coverage-v8 to v4

Bump vitest 3.2.4 → 4.0.18 and @vitest/coverage-v8 3.2.4 → 4.0.18 together,
since the coverage plugin requires a matching vitest peer dependency.

Closes #254 (dependabot PR that only bumped coverage-v8).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix(test): use class-based IntersectionObserver mock

The arrow-function mock (`vi.fn(() => ({...}))`) is not a valid constructor,
causing `new IntersectionObserver()` to throw in framer-motion and useParallax.
Replace with a proper class that implements the IntersectionObserver interface.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: Dbochman