feat: add background startup mode

[w3lld1]

Summary

• I added a --background option that starts Switchyard without opening the settings window and keeps the application alive for the session.
• I use GApplication registration to avoid activating an already-running primary instance, while allowing a later normal invocation to show the window.
• I documented native and Flatpak startup commands.

Validation

• go test -coverprofile=coverage.out ./internal/...
• go vet ./internal/...
• python3 scripts/generate-flatpak-go-modules.py followed by git diff --exit-code -- flatpak/go-modules.json
• npx prettier --check src/content/docs/using.md
• npm run lint
• npm run typecheck

I could not run the full GTK/Flatpak build locally because GTK4/libadwaita development tooling is unavailable in this environment. The website build reached Astro's image-optimization phase but did not complete before the local timeout.

Fixes #53

[alyraffauf]

I'll review this soon, but--on the off chance a human sees this--please do not have your agents write autonomous PRs. AI-assisted development is welcome here, but low quality contributions that don't even bother to build before pushing are absolutely slop.