test(frontend): cover mediaSrc, changelog, and timeline grouping

Unit tests for three previously-untested pure functions: mediaSrc's
UI Lab path handling, getChangelogForVersion's version normalization,
and groupImages' date bucketing/sorting.

src/components/timeline/timelineModel.test.ts

@@ -0,0 +1,40 @@
+import { describe, expect, it } from 'vitest'
+import { makeImage } from '../../test/factories'
+import { groupImages } from './timelineModel'
+
+describe('groupImages', () => {
+  it('buckets images by year-month, preferring taken_at over modified_at', () => {
+    const groups = groupImages([
+      makeImage({ id: 1, taken_at: '2026-03-05T00:00:00Z', modified_at: '2026-01-01T00:00:00Z' }),
+      makeImage({ id: 2, taken_at: null, modified_at: '2026-03-20T00:00:00Z' }),
+    ])
+
+    expect(groups).toHaveLength(1)
+    expect(groups[0].key).toBe('2026-03')
+    expect(groups[0].images.map((i) => i.id)).toEqual([1, 2])
+  })
+
+  it('sorts groups chronologically ascending', () => {
+    const groups = groupImages([
+      makeImage({ id: 1, taken_at: '2026-06-01T00:00:00Z' }),
+      makeImage({ id: 2, taken_at: '2026-01-01T00:00:00Z' }),
+      makeImage({ id: 3, taken_at: '2026-03-01T00:00:00Z' }),
+    ])
+
+    expect(groups.map((g) => g.key)).toEqual(['2026-01', '2026-03', '2026-06'])
+  })
+
+  it('buckets images with no date under "unknown" and sorts it last', () => {
+    const groups = groupImages([
+      makeImage({ id: 1, taken_at: null, modified_at: null }),
+      makeImage({ id: 2, taken_at: '2026-01-01T00:00:00Z' }),
+    ])
+
+    expect(groups.map((g) => g.key)).toEqual(['2026-01', 'unknown'])
+    expect(groups[1].label).toBe('Unknown Date')
+  })
+
+  it('returns no groups for an empty image list', () => {
+    expect(groupImages([])).toEqual([])
+  })
+})