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>

turbo.json

@@ -15,6 +15,10 @@
 		"test": {
 			"inputs": ["src/**", "vitest.config.*", "package.json"]
 		},
+		"test:coverage": {
+			"inputs": ["src/**", "vitest.config.*", "package.json"],
+			"outputs": ["coverage/**"]
+		},
 		"test:e2e": {
 			"cache": false,
 			"inputs": ["e2e/**", "src/**", "playwright.config.*", "package.json"]