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

vitest.config.ts

@@ -8,5 +8,23 @@ export default defineConfig({
     environment: "jsdom",
     setupFiles: ["./vitest.setup.ts"],
     globals: true,
+    coverage: {
+      provider: "v8",
+      reporter: ["text", "json", "html"],
+      reportsDirectory: "./coverage",
+      include: ["src/lib/**/*.ts"],
+      exclude: [
+        "src/lib/**/*.test.ts",
+        "src/lib/**/*.spec.ts",
+        "src/lib/**/*.d.ts",
+        "src/lib/components/**",
+      ],
+      thresholds: {
+        statements: 15,
+        branches: 13,
+        functions: 13,
+        lines: 15,
+      },
+    },
   },
 });