Late April Fixes

Another round of correctness and polish fixes across the platform:

• Checkout for V2 users — buyers coming from legacy V2 accounts can once again complete checkout on public offer pages; their tenant record is created on success instead of blocking the purchase
• Locked module attachments — when a module is locked behind prerequisites, its attachments (files, videos, links) are now visually dimmed and gated like the rest of the module, and clicking them opens the required-modules modal instead of the resource
• Trial billing display — the billing page now shows “Trial Ends” with the trial end date while a subscription is trialing, instead of mislabeling that date as “Next Payment”
• Dropdown overflow — long dropdown menus (such as language or option pickers) now scroll within the viewport instead of getting cut off on shorter screens