test(frontend): add Vitest unit tests for store helpers and utilities

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.

package.json

@@ -23,7 +23,10 @@
     "format:rust:check": "cd src-tauri && cargo fmt --check",
     "preview": "vite preview",
     "tauri": "tauri",
-    "test:e2e": "playwright test"
+    "test:e2e": "playwright test",
+    "test:rust": "cd src-tauri && cargo test --no-default-features",
+    "test:unit": "vitest run",
+    "test:unit:watch": "vitest"
   },
   "dependencies": {
     "@tanstack/react-virtual": "^3.13.23",
@@ -53,6 +56,7 @@
     "prettier-plugin-tailwindcss": "^0.8.0",
     "tailwindcss": "^4.2.2",
     "typescript": "~5.8.3",
-    "vite": "^7.0.4"
+    "vite": "^7.0.4",
+    "vitest": "^4.1.9"
   }
 }