Invoice downloads and free-plan header banner

Two billing touch-ups landed in the December release.

• Each invoice row on the Billing page now exposes a download icon that fetches a signed PDF link and triggers a browser download — no more requesting copies through support.
• A dismissable header banner is shown to free-plan users with a CTA that jumps directly to the Plans page; the banner respects the user’s preference between sessions.

Both surfaces hook into the centralized invoice and plan APIs, so they stay accurate even when plan changes happen mid-cycle.