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/package.json

@@ -7,6 +7,7 @@
     "dev": "vite",
     "build": "vite build",
     "typecheck": "tsc -p tsconfig.json --noEmit",
+    "test": "bun test",
     "preview": "vite preview",
     "lint": "biome lint src",
     "lint:fix": "biome lint --write src",
@@ -60,6 +61,7 @@
   },
   "devDependencies": {
     "@tailwindcss/vite": "^4.1.18",
+    "@types/bun": "^1.3.4",
     "@types/react": "^18.3.0",
     "@types/react-dom": "^18.3.0",
     "@vitejs/plugin-react": "^4.3.0",