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_dashboard.yaml

@@ -21,6 +21,12 @@ jobs:
         with:
           node-version: '24'
 
+      - uses: actions/cache@v4
+        with:
+          path: ~/.npm
+          key: dashboard-npm-${{ hashFiles('apps/dashboard-ui/package-lock.json') }}
+          restore-keys: dashboard-npm-
+
       - name: Install
         working-directory: apps/dashboard-ui
         run: npm ci || npm install