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/vitest.config.ts

@@ -22,5 +22,26 @@ export default defineConfig({
 		css: false,
 		include: ["src/**/*.{test,spec}.{ts,tsx}"],
 		exclude: ["node_modules", "dist", "e2e"],
+		coverage: {
+			provider: "v8",
+			reporter: ["text", "html", "json-summary", "lcov"],
+			include: ["src/**/*.{ts,tsx}"],
+			exclude: [
+				"src/**/*.{test,spec}.{ts,tsx}",
+				"src/test/**",
+				"src/routeTree.gen.ts",
+				"src/api/schema.d.ts",
+				"src/main.tsx",
+				"src/vite-env.d.ts",
+			],
+			// Truthful baseline floor measured on main (not copied from the
+			// fork). Ratchet upward as feature tests grow.
+			thresholds: {
+				lines: 18,
+				functions: 10,
+				branches: 10,
+				statements: 17,
+			},
+		},
 	},
 });