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/tsconfig.json

@@ -25,7 +25,7 @@
     "paths": {
       "@/*": ["./src/*"]
     },
-    "types": ["vite/client"]
+    "types": ["vite/client", "bun"]
   },
   "include": ["src"],
   "references": [{ "path": "./tsconfig.node.json" }]