Add login link to plan/product selection modals on default UI

🔍 Problem Statement

Users who land on the plan/product selection modals without being logged in have no clear path to authenticate. This creates friction in the subscription flow, as users may abandon the process instead of figuring out how to log in and continue.

💡 User Story

As a returning user,

I want to be able to log in directly from the plan/product selection modal,

so that I can quickly access my account and complete my subscription without restarting the flow.

🎯 Definition of Done (DoD)

A feature is done when:

A feature is considered done when:

✔️ Add the text “Don’t have an account? Sign up or log in” to the bottom of both product and plan selection modals (aligned with the registration modal design)

✔️ “Sign up” opens the registration modal

✔️ “Log in” opens the sign-in modal

✔️ UI/UX is consistent with existing authentication modals (styling, spacing, behavior)

✔️ No regression in the subscription / checkout flow