if mempool fee suggestion is not available: do not use 1 sat/vB

[myxmaster]

Describe your enhancement idea

In case mempool fee suggestion is enabled but not available (e.g. mempool API down, easy to simulate by editing URL in FeeStore.ts:56), Zeus uses 1 sat/vB (or purge threshold?) which can be very annoying of course.

I found 2 relevant situations where we should improve UI:

1. Sending on-chain:

• a warning is displayed 👍
• user cannot see that a feerate of 1 sat/vB is set 👎
• if user doesn't click on the warning but just sends, 1 sat/vB is used 👎

2. Channel opening:

• no warning is displayed (hidden in 'Advanced settings') 👎
• user cannot see that a feerate of 1 sat/vB is set 👎
• If user doesn't click on the warning but just sents, 1 sat/vB is used 👎

3. Channel closing:

• a warning is displayed 👍
• user cannot see that a feerate of 1 sat/vB is set 👎
• if user doesn't click on the warning but just sends, 1 sat/vB is used 👎

So for channel openings we need to display a warning, and for all cases we should force the user to enter a custom fee rate (disable Send/Open/Close buttons?).