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/vitest.config.ts

@@ -19,5 +19,13 @@ export default defineConfig({
     // collected, and the gate still prints green.
     include: ["src/**/*.test.{ts,tsx}"],
     environment: "node",
+    // E32 D4: a coverage REPORT, read and not enforced — the gate's `vitest --coverage` prints the
+    // summary; no threshold, because "every regression test seen red" is the bar, not a percentage.
+    coverage: {
+      provider: "v8",
+      include: ["src/**/*.{ts,tsx}"],
+      exclude: ["src/**/*.test.{ts,tsx}", "src/**/*.testkit.ts", "src/**/*.d.ts"],
+      reporter: ["text-summary"],
+    },
   },
 });