test: a size and complexity ratchet; coverage reported on both sides

The "class size" item from the quality checklist, measured:
- A shipped file over 800 lines may not grow past its line count today
  (13 frontend, 11 backend). No other file may cross 800.
- A function over the complexity limit may not rise, and no new one may
  appear. The limit is eslint `complexity` 20 on the frontend (64 functions
  over it today) and ruff C901 10 on the backend (32).
- Each is a per-file ceiling. Shrinking is free; an entry back under the
  limit must be deleted, so the lists only shrink.

C901 is measured in a test rather than switched on in ruff.toml: ruff has no
per-function baseline, and a noqa would let a function grow without limit.

Coverage (owner decision D4) is printed with no threshold:
`vitest --coverage` (v8) gives 80% of lines, and the pytest lane ends with
the total for app/, 78%. The bar stays "every regression test seen red".
.coverage is ignored by git and kept out of the image's build context.

frontend/package.json

@@ -49,6 +49,7 @@
     "@types/react": "19.3.0",
     "@types/react-dom": "19.3.0",
     "@vitejs/plugin-react": "^4.3.1",
+    "@vitest/coverage-v8": "4.1.11",
     "@vitest/eslint-plugin": "1.6.27",
     "babel-plugin-react-compiler": "1.0.0",
     "eslint": "9.39.1",