ci: install cmake + ninja-build (act-runner image is slim)

GitHub-hosted ubuntu-latest preinstalls cmake and ninja. Gitea's
act-runner uses a minimal Ubuntu image (catthehacker/ubuntu:act-*)
which doesn't, so the build step fails with "cmake: command not
found" (exit 127).

apt-get update was already run by install-qt-action's deps step
earlier in the job, so the lists are populated — just install.

Same step added to ci.yml and release.yml.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -50,6 +50,9 @@ jobs:
         with:
           python-version: '3.12'
 
+      - name: Install build tools (cmake, ninja)
+        run: sudo apt-get install -y cmake ninja-build
+
       - name: Setup Qt 6
         uses: jurplel/install-qt-action@v4
         with:

.gitea/workflows/release.yml

@@ -33,6 +33,9 @@ jobs:
         with:
           python-version: '3.12'
 
+      - name: Install build tools (cmake, ninja)
+        run: sudo apt-get install -y cmake ninja-build
+
       - name: Setup Qt 6
         uses: jurplel/install-qt-action@v4
         with: