feat: stop offering new lightning channels and warn of channel closing #1669

Twin: synonymdev/bitkit-ios#1097

Why

Synonym is winding down Blocktank. The first step is that the app stops offering new Lightning channels from Blocktank and tells users their existing channels will be closed. Existing channels keep working until then: sending, receiving over an open channel and transfers to savings stay as they are.

Scope

1. Remove every path that buys a new channel from Blocktank. Known entry points:
   • transfer to spending (savings → spending, channel order and payment)
   • instant receive (CJIT): the receive sheet no longer offers a Lightning invoice that needs a new channel; a user with no usable channel gets an on-chain address only
   • gift codes, which create a channel order
   • any other screen that calls Blocktank order or CJIT creation (onboarding, advanced channel setup, dev screens excepted)
2. Notice to users who have Lightning channels: a visible message that their Lightning channels will be closed on a set date, and to move their spending balance to savings before then. The date is one value that is set before release.
3. Users without channels see no Lightning purchase or CJIT prompt anywhere.

Out of scope

• Closing channels: Blocktank closes them later, after about six weeks.
• Stopping orders on Blocktank's server.
• Any replacement for Lightning (swaps, other providers).

Acceptance

• No flow in the app can create a Blocktank channel order or CJIT entry.
• Existing channels: send, receive and transfer to savings still work.
• The notice shows for every user with an open channel, with the date.