test(web): add v8 coverage with truthful baseline floor

Adds @vitest/coverage-v8 (catalog-pinned), a test:coverage script wired
through turbo (inputs + coverage/** output), and coverage thresholds set
to a floor measured on main (lines 18 / funcs 10 / branches 10 / stmts
17 against current 19.3/11.4/11.7/18.9). Ratchet upward as tests grow.

Adapts the coverage config from @BenSheridanEdwards's fork; the multi-job
CI fan-out from that branch was intentionally left out.

Co-authored-by: Ben Sheridan-Edwards <BenSheridanEdwards@users.noreply.github.com>

packages/web/package.json

@@ -10,6 +10,7 @@
 		"lint": "biome check src/",
 		"lint:fix": "biome check --write src/",
 		"test": "vitest run --passWithNoTests",
+		"test:coverage": "vitest run --coverage",
 		"test:e2e": "playwright test",
 		"generate:api": "openapi-typescript openapi.json -o src/api/schema.d.ts"
 	},
@@ -54,6 +55,7 @@
 		"@types/react": "catalog:",
 		"@types/react-dom": "catalog:",
 		"@vitejs/plugin-react": "catalog:",
+		"@vitest/coverage-v8": "catalog:",
 		"jsdom": "catalog:",
 		"openapi-typescript": "^7.8.0",
 		"typescript": "catalog:",