ci: add coverage tooling for frontend and backend

- Add @vitest/coverage-v8 for frontend test coverage
- Configure vitest with coverage thresholds (15% statements/lines, 13% branches/functions)
- Add npm scripts for backend testing and coverage with cargo-llvm-cov
- Add test:all and coverage:all scripts for unified test execution
- Exclude coverage directory from git tracking

package.json

@@ -16,6 +16,10 @@
     "test": "vitest run",
     "test:watch": "vitest",
     "test:coverage": "vitest run --coverage",
+    "test:backend": "cd src-tauri && cargo test",
+    "test:backend:coverage": "cd src-tauri && cargo llvm-cov --text",
+    "test:all": "pnpm test && pnpm test:backend",
+    "coverage:all": "pnpm test:coverage && pnpm test:backend:coverage",
     "lint": "eslint .",
     "lint:fix": "eslint . --fix",
     "format": "prettier --write .",
@@ -44,6 +48,7 @@
     "@tauri-apps/cli": "^2",
     "@testing-library/jest-dom": "^6.9.1",
     "@testing-library/svelte": "^5.3.1",
+    "@vitest/coverage-v8": "^4.0.18",
     "eslint": "^9.39.2",
     "eslint-config-prettier": "^10.1.8",
     "eslint-plugin-svelte": "^3.14.0",