ci: add Gitea-compatible caching (RUNNER_TOOL_CACHE + actions/cache@v3 + go-hashfiles)

Per Gitea tutorial: add RUNNER_TOOL_CACHE=/toolcache env, use actions/cache@v3
(not v4), and go-hashfiles instead of hashFiles() which is unsupported.

.gitea/workflows/build_dashboard.yaml

@@ -6,6 +6,8 @@ on:
 jobs:
   build:
     runs-on: ubuntu-latest
+    env:
+      RUNNER_TOOL_CACHE: /toolcache
     steps:
       - name: Configure git auth for submodules
         run: git config --global url."https://x-access-token:${{ secrets.BUILD_TOKEN }}@git.datadunia.com/".insteadOf "https://git.datadunia.com/"
@@ -21,6 +23,19 @@ jobs:
         with:
           node-version: '24'
 
+      - uses: https://gitea.com/actions/go-hashfiles@v0.0.1
+        id: hash-npm
+        with:
+          patterns: |
+            apps/dashboard-ui/package-lock.json
+
+      - uses: actions/cache@v3
+        with:
+          path: |
+            ~/.npm
+          key: npm-dashboard-${{ steps.hash-npm.outputs.hash }}
+          restore-keys: npm-dashboard-
+
       - name: Install
         working-directory: apps/dashboard-ui
         run: npm ci || npm install