ci(workflows): improve caching and optimize dependency installation

Add Next.js build cache to deployment workflow for improved performance. Update all workflows to use `pnpm install --frozen-lockfile --prefer-offline` for faster and more reliable dependency management.

.gitea/workflows/backend-tests.yml

@@ -31,6 +31,6 @@ jobs:
           restore-keys: |
             ${{ runner.os }}-pnpm-store-
       - name: Install dependencies
-        run: pnpm install --frozen-lockfile
+        run: pnpm install --frozen-lockfile --prefer-offline
       - name: Run Backend Tests
         run: pnpm -F @memegoat/backend test