Wire Vitest into the existing Vite config (scoped to src/**/*.test.ts so it
never collides with the Playwright suite in tests/) and add test:unit,
test:unit:watch, and test:rust scripts.
Covers the pure logic layer: parseSearchValue prefix parsing, all twelve
sort orders through mergeImages, merge/dedup/window helpers, filter
matching, gallery request tokens, localStorage-backed initial settings,
folder-picker path utilities, and the duplicate/lightbox/gallery/video
formatters. Fixture factories live in src/test/factories.ts.
