[test] Baseline test: Re-add .cache to measure performance WITH cache

This commit temporarily restores .cache caching to establish a baseline
for performance comparison. The next commit will remove it again to
measure the performance impact.

Testing approach:
1. This commit: WITH .cache (baseline)
2. Next commit: WITHOUT .cache (comparison)
3. Analysis: Compare CI run times to quantify cache impact

.github/workflows/ci-tests-unit.yaml

@@ -33,6 +33,7 @@ jobs:
         uses: actions/cache@v4
         with:
           path: |
+            .cache
             coverage
             .vitest-cache
           key: vitest-cache-${{ runner.os }}-${{ hashFiles('**/pnpm-lock.yaml') }}-${{ hashFiles('src/**/*.{ts,vue,js}', 'vitest.config.*', 'tsconfig.json') }}