fix(Earn): validate offer minsize

[theborakompanioni]

Fixes #740.
However, this is a temporary fix to prevent the backend from dying.

There should be a follow-up PR that adds better explanation more prominently.
e.g. similar to the CoinjoinRequirementSummary/CoinjoinRequirementViolation alerts on "Jam" and "Send".

[editwentyone]

any hint why the lowest is 27300 sats? but other than that, approved

[theborakompanioni]

any hint why the lowest is 27300 sats? but other than that, approved

27 300 is the "dusty minsize" and apparently needed to be substracted from the minsize. I am totally sure as to why that is the case. However, the backend does not like larger values and dies a while later.

Have you reproduced the problem and verified that this PR prevents the issue yourself?

[editwentyone]

double checked, reproduced, now double approved

[theborakompanioni]

double checked, reproduced, now double approved

Thank you 🙏