ci: use org runner group Default

[fabianzimber]

What this changes

• All GitHub Actions jobs now target the organization self-hosted runner group Default (Linux x64):

runs-on:
  group: Default
  labels: [self-hosted, linux, x64]

Decisions

• Replaced ubuntu-*, macos-*, and matrix-based runs-on: ${{ matrix.os }} with the runner group mapping above.
• No changes to permissions, steps, or secrets.
• Notable runner requirements:
  • Tauri macOS build/smoke jobs: packaging a macOS app typically requires macOS tooling (e.g., Xcode). If Default is Linux-only, these jobs will need a macOS self-hosted runner group or the job logic adjusted.
  • Docker services: not used in current workflows.
  • CodeQL: not used in current workflows.
  • Playwright: not used in current workflows.

Validation

• Static YAML edits only; no application code changes.
• Verified no job under .github/workflows/ still uses a GitHub-hosted runs-on string.