ci: consolidate test jobs into single workflow with npm caching

- Merge 3 parallel test jobs (backend/frontend/mobile) into one sequential job
- Remove dependency on dorny/paths-filter@v3 (replaced with git diff script)
- Add npm cache via setup-node to speed up dependency installation
- Downgrade Node.js from 24 to 22 LTS for stability
- Reduce checkout fetch-depth from full history to 2 commits

.gitea/workflows/docker.yaml

@@ -36,7 +36,7 @@ jobs:
     - name: Setup Node.js
       uses: actions/setup-node@v4
       with:
-        node-version: '24'
+        node-version: '22'
         cache: 'npm'
     - name: Install dependencies
       run: npm ci
@@ -53,7 +53,7 @@ jobs:
     - name: Setup Node.js
       uses: actions/setup-node@v4
       with:
-        node-version: '24'
+        node-version: '22'
         cache: 'npm'
     - name: Install dependencies
       run: npm ci