Polish community onboarding flow

[cynfria]

Summary

• Restyle first-community setup as three stacked scenario cards with shared onboarding footer controls.
• Restyle request-access, invite-link, and profile setup screens to match the step-one key container treatment.
• Add back navigation from community setup into machine config and regression coverage for key/url frame positioning.

Test plan

• bin/pnpm --dir desktop build
• bin/pnpm --dir desktop exec playwright test --project=integration tests/e2e/onboarding.spec.ts -g "first-community choices expose npub and invite input|first-community shows the scenario cards for localhost|first-community profile step uses onboarding Next and Back controls"
• bin/pnpm --dir desktop check:px-text
• git diff --check
• pre-push hook suite via git push