ci: add actions/cache@v4 for Go modules, Go build cache, and npm cache

- server-core: Go module + build cache (keyed by go.sum)
- dashboard-ui: npm cache (keyed by package-lock.json)
- device-agent: Go module + build cache (keyed by go.sum)
First run populates cache, subsequent runs restore from cache.

.gitea/workflows/build_device_agent.yaml

@@ -21,6 +21,14 @@ jobs:
         with:
           go-version: '1.26'
 
+      - uses: actions/cache@v4
+        with:
+          path: |
+            ~/go/pkg/mod
+            ~/.cache/go-build
+          key: device-agent-go-${{ hashFiles('apps/device-agent/go.sum') }}
+          restore-keys: device-agent-go-
+
       - name: Build all platforms
         working-directory: apps/device-agent
         shell: bash