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/package.json

@@ -14,6 +14,7 @@
 		"lint": "prettier --check . && eslint .",
 		"test:unit": "vitest",
 		"test": "npm run test:unit -- --run",
+		"test:coverage": "vitest run --coverage --project=server",
 		"test:e2e": "playwright test",
 		"test:e2e:headed": "playwright test --headed",
 		"test:e2e:ui": "playwright test --ui",
@@ -38,6 +39,7 @@
 		"@types/diff": "^7.0.2",
 		"@types/node": "^24",
 		"@vitest/browser-playwright": "^4.0.10",
+		"@vitest/coverage-v8": "^4.1.0",
 		"eslint": "^9.39.1",
 		"eslint-config-prettier": "^10.1.8",
 		"eslint-plugin-svelte": "^3.13.0",