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

.gitignore

@@ -8,3 +8,6 @@ node_modules
 !.env.example
 vite.config.js.timestamp-*
 vite.config.ts.timestamp-*
+
+# Coverage reports
+/coverage