feat: ask to include AI subscription on signup

[dimavedenyapin]

Summary

• routes the dashboard cloud connect CTA through the existing 20x Cloud modal so signup can ask one more question before opening the browser
• adds a default-on "Include AI subscription" checkbox for browser registration and passes the choice through renderer, preload, and main IPC
• appends include_ai_subscription=true to workflow-builder /register URLs when selected

Test plan

• pnpm test:run --project renderer src/renderer/src/components/settings/tabs/EnterpriseLoginModal.test.tsx src/renderer/src/components/dashboard/DashboardWorkspace.test.tsx src/renderer/src/stores/enterprise-store.test.ts
• pnpm test:run --project renderer
• pnpm typecheck
• pnpm lint
• pnpm test:run (fails in main suites because better-sqlite3 native binding is missing after rebuild failure under local Node 26; renderer suites passed)