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.

vite.config.ts

@@ -1,3 +1,4 @@
+/// <reference types="vitest/config" />
 import { defineConfig } from 'vite'
 import react from '@vitejs/plugin-react'
 import tailwindcss from '@tailwindcss/vite'
@@ -9,6 +10,11 @@ const host = process.env.TAURI_DEV_HOST
 export default defineConfig(async () => ({
   plugins: [react(), tailwindcss()],
 
+  test: {
+    include: ['src/**/*.test.ts'],
+    environment: 'node',
+  },
+
   clearScreen: false,
   server: {
     port: 1420,