Qt CI on GitHub Actions

Installing Qt takes longer than compiling the application.

Why it happens

aqtinstall downloads several gigabytes of Qt on every run with no cache.

What to change

1. Cache the Qt installation directory keyed on the Qt version
2. Use ccache for the C++ compilation itself
3. Build with ninja and full parallelism

Qt on runnerhut

jobs:
  build:
    runs-on: runnerhut-16vcpu-ubuntu-2404
    steps:
      - uses: actions/checkout@v4
      - uses: jurplel/install-qt-action@v4
        with: { version: '6.8.0', cache: true }
      - run: cmake -B build -G Ninja && cmake --build build --parallel

The only runnerhut-specific line is `runs-on: runnerhut-16vcpu-ubuntu-2404`. Everything else is standard GitHub Actions — the same actions, the same secrets and the same permissions model you use today.