test(app): add bun test infra and first unit tests

bun test as the runner — already the toolchain, zero new deps beyond
@types/bun. 54 tests across 5 files covering the already-pure logic:
FastAPI error normalization (extracted verbatim to lib/api/errors.ts),
clip trim clamping (extracted from StoryTrackEditor to lib/utils/trim.ts,
magic 100 now MIN_CLIP_DURATION_MS), duration/size/engine formatters,
engine-language map consistency, and changelog parsing. Wired into CI
after typecheck.

Known issues surfaced by the tests, left as-is for now: parseChangelog's
heading regex uses \s* which matches newlines, so a dateless heading
followed directly by a bullet swallows that bullet; formatFileSize has
no TB unit; ENGINE_DISPLAY_NAMES lacks tada/kokoro.

app/src/lib/utils/format.test.ts

@@ -0,0 +1,71 @@
+import { describe, expect, test } from 'bun:test';
+import { formatDuration, formatEngineName, formatFileSize } from './format';
+
+describe('formatDuration', () => {
+  test('formats zero', () => {
+    expect(formatDuration(0)).toBe('0:00');
+  });
+
+  test('pads single-digit seconds', () => {
+    expect(formatDuration(65)).toBe('1:05');
+  });
+
+  test('handles the minute boundary', () => {
+    expect(formatDuration(59)).toBe('0:59');
+    expect(formatDuration(60)).toBe('1:00');
+  });
+
+  test('floors fractional seconds', () => {
+    expect(formatDuration(89.9)).toBe('1:29');
+  });
+
+  test('does not roll minutes into hours', () => {
+    expect(formatDuration(3661)).toBe('61:01');
+  });
+});
+
+describe('formatFileSize', () => {
+  test('special-cases zero', () => {
+    expect(formatFileSize(0)).toBe('0 Bytes');
+  });
+
+  test('formats bytes below 1 KB', () => {
+    expect(formatFileSize(512)).toBe('512 Bytes');
+    expect(formatFileSize(1023)).toBe('1023 Bytes');
+  });
+
+  test('formats KB, MB, and GB boundaries', () => {
+    expect(formatFileSize(1024)).toBe('1 KB');
+    expect(formatFileSize(1024 ** 2)).toBe('1 MB');
+    expect(formatFileSize(1024 ** 3)).toBe('1 GB');
+  });
+
+  test('rounds to two decimal places', () => {
+    expect(formatFileSize(1536)).toBe('1.5 KB');
+    expect(formatFileSize(2_684_354_560)).toBe('2.5 GB');
+    expect(formatFileSize(1_234_567)).toBe('1.18 MB');
+  });
+});
+
+describe('formatEngineName', () => {
+  test('maps known engines to display names', () => {
+    expect(formatEngineName('luxtts')).toBe('LuxTTS');
+    expect(formatEngineName('chatterbox')).toBe('Chatterbox');
+    expect(formatEngineName('chatterbox_turbo')).toBe('Chatterbox Turbo');
+  });
+
+  test('defaults to Qwen when engine is undefined', () => {
+    expect(formatEngineName()).toBe('Qwen');
+    expect(formatEngineName(undefined, '1.7B')).toBe('Qwen');
+  });
+
+  test('appends the model size for qwen only', () => {
+    expect(formatEngineName('qwen', '1.7B')).toBe('Qwen 1.7B');
+    expect(formatEngineName('qwen')).toBe('Qwen');
+    expect(formatEngineName('luxtts', '1.7B')).toBe('LuxTTS');
+  });
+
+  test('passes unknown engines through verbatim', () => {
+    expect(formatEngineName('kokoro')).toBe('kokoro');
+  });
+});