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.

bun.lock

@@ -65,6 +65,7 @@
       },
       "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",
@@ -611,6 +612,8 @@
 
     "@types/babel__traverse": ["@types/[email protected]", "", { "dependencies": { "@babel/types": "^7.28.2" } }, "sha512-8PvcXf70gTDZBgt9ptxJ8elBeBjcLOAcOtoO/mPJjtji1+CdGbHgm77om1GrsPxsiE+uXIpNSK64UYaIwQXd4Q=="],
 
+    "@types/bun": ["@types/[email protected]", "", { "dependencies": { "bun-types": "1.3.14" } }, "sha512-h1hFqFVcvAvD9j9K7ZW7vd82aSA+rTdznZa+5bwvCwqSB1jmmfLcbIWhOLx1/+boy/xmjgCs/OMUL8hRJSmnPw=="],
+
     "@types/estree": ["@types/[email protected]", "", {}, "sha512-dWHzHa2WqEXI/O1E9OjrocMTKJl2mSrEolh1Iomrv6U+JuNwaHXsXx9bLu5gG7BUWFIN0skIQJQ/L1rIex4X6w=="],
 
     "@types/node": ["@types/[email protected]", "", { "dependencies": { "undici-types": "~6.21.0" } }, "sha512-WJtwWJu7UdlvzEAUm484QNg5eAoq5QR08KDNx7g45Usrs2NtOPiX8ugDqmKdXkyL03rBqU5dYNYVQetEpBHq2g=="],
@@ -685,6 +688,8 @@
 
     "browserslist": ["[email protected]", "", { "dependencies": { "baseline-browser-mapping": "^2.9.0", "caniuse-lite": "^1.0.30001759", "electron-to-chromium": "^1.5.263", "node-releases": "^2.0.27", "update-browserslist-db": "^1.2.0" }, "bin": { "browserslist": "cli.js" } }, "sha512-ZC5Bd0LgJXgwGqUknZY/vkUQ04r8NXnJZ3yYi4vDmSiZmC/pdSN0NbNRPxZpbtO4uAfDUAFffO8IZoM3Gj8IkA=="],
 
+    "bun-types": ["[email protected]", "", { "dependencies": { "@types/node": "*" } }, "sha512-4N0ig0fEomHt5R0KCFWjovxow98rIoRwKolrYdCcknNwMekCXRnWEUvgu5soYV8QXtVsrUD8B95MBOZGPvr6KQ=="],
+
     "callsites": ["[email protected]", "", {}, "sha512-P8BjAsXvZS+VIDUI11hHCQEv74YT67YUi5JJFNWIqL235sBmjX4+qx9Muvls5ivyNENctx46xQLQ3aTuE7ssaQ=="],
 
     "camelcase-css": ["[email protected]", "", {}, "sha512-QOSvevhslijgYwRx6Rv7zKdMF8lbRmx+uQGx2+vDc+KI/eBnsy9kit5aj23AgGu3pa4t9AgwbnXWqS+iOY+2aA=="],