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.

package.json

@@ -23,6 +23,7 @@
     "update:icons": "./scripts/update-icons.sh",
     "convert:assets": "./scripts/convert-assets.sh",
     "lint": "biome lint .",
+    "test": "cd app && bun test",
     "typecheck": "bunx tsc -p app/tsconfig.json --noEmit && cd web && bunx tsc --noEmit",
     "lint:fix": "biome lint --write .",
     "format": "biome format --write .",