test(#121): add @vitest/coverage-v8 with 80% branch coverage gate

Installs @vitest/coverage-v8 and configures coverage measurement over
src/lib/utils/** and src/lib/server/** — the utility and server-side
logic that is meaningful to measure in the Node test project.

Svelte component files and generated code (api/**, paraglide/**) are
excluded; those run in the browser project.

Baseline: 87.87% branch coverage — already above the 80% threshold.
Adds test:coverage script for local runs; produces lcov report for CI.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/vite.config.ts

@@ -43,6 +43,16 @@ export default defineConfig({
 	],
 	test: {
 		expect: { requireAssertions: true },
+		coverage: {
+			provider: 'v8',
+			reporter: ['text', 'lcov'],
+			// Measure utility and server-side logic only — Svelte components
+			// run in the browser project and are excluded here intentionally.
+			include: ['src/lib/utils/**', 'src/lib/server/**'],
+			thresholds: {
+				branches: 80
+			}
+		},
 		projects: [
 			{
 				extends: './vite.config.ts',