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/components/duplicateFinder/format.test.ts

@@ -0,0 +1,61 @@
+import { describe, expect, it } from 'vitest'
+import type { DuplicateScanProgress } from '../../store'
+import {
+  duplicateProgressLabel,
+  duplicateProgressPercent,
+  formatBytes,
+  formatRelativeTime,
+} from './format'
+
+function progress(overrides: Partial<DuplicateScanProgress> = {}): DuplicateScanProgress {
+  return { phase: 'checking', processed: 0, total: 0, skipped: 0, ...overrides }
+}
+
+describe('formatBytes', () => {
+  it('formats each size tier', () => {
+    expect(formatBytes(0)).toBe('0 B')
+    expect(formatBytes(512)).toBe('512 B')
+    expect(formatBytes(2048)).toBe('2 KB')
+    expect(formatBytes(1_048_576)).toBe('1.0 MB')
+    expect(formatBytes(1_610_612_736)).toBe('1.5 GB')
+  })
+})
+
+describe('formatRelativeTime', () => {
+  const now = () => Math.floor(Date.now() / 1000)
+
+  it('formats each time bucket', () => {
+    expect(formatRelativeTime(now())).toBe('just now')
+    expect(formatRelativeTime(now() - 120)).toBe('2m ago')
+    expect(formatRelativeTime(now() - 7200)).toBe('2h ago')
+    expect(formatRelativeTime(now() - 172_800)).toBe('2d ago')
+  })
+})
+
+describe('duplicateProgressLabel', () => {
+  it('returns null without progress', () => {
+    expect(duplicateProgressLabel(null)).toBeNull()
+  })
+
+  it('labels each phase', () => {
+    expect(duplicateProgressLabel(progress({ phase: 'checking' }))).toBe('Checking file sizes')
+    expect(duplicateProgressLabel(progress({ phase: 'hashing' }))).toBe(
+      'Hashing duplicate candidates'
+    )
+    expect(duplicateProgressLabel(progress({ phase: 'confirming' }))).toBe(
+      'Confirming exact matches'
+    )
+  })
+})
+
+describe('duplicateProgressPercent', () => {
+  it('returns 0 for missing progress or zero totals', () => {
+    expect(duplicateProgressPercent(null)).toBe(0)
+    expect(duplicateProgressPercent(progress({ processed: 5, total: 0 }))).toBe(0)
+  })
+
+  it('rounds to whole percent', () => {
+    expect(duplicateProgressPercent(progress({ processed: 50, total: 200 }))).toBe(25)
+    expect(duplicateProgressPercent(progress({ processed: 1, total: 3 }))).toBe(33)
+  })
+})