Keep setup controls visible at minimum size

[enaboapps]

Closes #660

Summary

• make the setup dialog a bounded flex column at the supported minimum window size
• keep the header, progress indicator, and footer fixed while only setup content scrolls
• remove the mobile content minimum height that caused nested overflow

Validation

• npm run lint
• npm test (74 frontend tests and 5 updater-feed tests)
• npm run build
• full app, updater verifier, and startup launcher formatting, Clippy, and tests
• npm audit and cargo audit 0.22.2 for all three lockfiles
• unsigned Windows UIAccess NSIS build and package verification
• in-app browser smoke test of all five setup steps at 620x520 and 920x700
• verified footer visibility, content-only overflow, forward/backward focus wrapping, and zero console warnings/errors