Late April Improvements

Another round of correctness and polish improvements across the platform:

• Checkout for V2 users: buyers with a legacy V2 account can complete checkout on public offer pages, and their tenant record is created when the purchase succeeds.
• Locked module attachments: when a module is locked behind prerequisites, its attachments (files, videos, links) are dimmed and gated like the rest of the module. Clicking one opens the required-modules modal.
• Trial billing display: while a subscription is trialing, the billing page shows “Trial Ends” with the trial end date.
• Dropdown menus: long dropdowns, such as language or option pickers, scroll within the viewport on shorter screens.