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.

src/test/factories.ts

@@ -0,0 +1,73 @@
+import type { Folder, FolderJobProgress, ImageRecord } from '../store/types'
+
+let nextImageId = 1
+
+export function makeImage(overrides: Partial<ImageRecord> = {}): ImageRecord {
+  const id = overrides.id ?? nextImageId++
+  return {
+    id,
+    folder_id: 1,
+    path: `C:/media/image-${id}.jpg`,
+    filename: `image-${id}.jpg`,
+    thumbnail_path: null,
+    width: 1920,
+    height: 1080,
+    file_size: 1024,
+    created_at: null,
+    modified_at: '2026-01-01T00:00:00Z',
+    taken_at: null,
+    mime_type: 'image/jpeg',
+    media_kind: 'image',
+    duration_ms: null,
+    video_codec: null,
+    audio_codec: null,
+    metadata_updated_at: null,
+    metadata_error: null,
+    favorite: false,
+    rating: 0,
+    embedding_status: 'pending',
+    embedding_model: null,
+    embedding_updated_at: null,
+    embedding_error: null,
+    generated_caption: null,
+    caption_model: null,
+    caption_updated_at: null,
+    caption_error: null,
+    ai_rating: null,
+    ai_tagger_model: null,
+    ai_tagged_at: null,
+    ai_tagger_error: null,
+    ...overrides,
+  }
+}
+
+export function makeFolder(overrides: Partial<Folder> = {}): Folder {
+  return {
+    id: 1,
+    path: 'C:/media',
+    name: 'media',
+    image_count: 0,
+    indexed_at: null,
+    scan_error: null,
+    sort_order: 0,
+    ...overrides,
+  }
+}
+
+export function makeFolderProgress(overrides: Partial<FolderJobProgress> = {}): FolderJobProgress {
+  return {
+    folder_id: 1,
+    thumbnail_pending: 0,
+    metadata_pending: 0,
+    embedding_pending: 0,
+    embedding_ready: 0,
+    embedding_failed: 0,
+    caption_pending: 0,
+    caption_ready: 0,
+    caption_failed: 0,
+    tagging_pending: 0,
+    tagging_ready: 0,
+    tagging_failed: 0,
+    ...overrides,
+  }
+}