Validate order amount against the node's min/max sats limits before submitting

[Catrya]

The create-order form only checks that amounts are positive and min < max
(_checkValid in lib/features/order/screens/add_order_screen.dart). It never
compares the amount against the Mostro node's min_order_amount /
max_order_amount, even though the app already parses both from the node info
event (they are shown in the About screen). An out-of-range order is only
rejected by the daemon after submission.

Expected behavior (v1 parity)

The mobile app validates before submitting (_validateSatsRange in
add_order_screen.dart): it converts the fexchange
rate, compares against the node limits, andalized
message like:

The amount is too low. This Mostro node anSats - maxSats sats).

with a sats-only fallback when the fiat ranth
bounds of a range order validated.

Notes

• For fixed-sats orders the check is direct
• For market-price orders appv2 currently h*
  (see comment in add_order_screen.dart a this
  needs porting an exchange-rate provider (s issue
  to fixed-sats first.
• Strings must be localized in the 5 .arb

[codaMW]

#282 done and up. Validates fixed-sats order amounts against the node's min/max before submitting, so out-of-range orders are caught client-side instead of by the daemon. Verified on my Nokia. Scoped to fixed-sats per the issue; market-price validation needs the exchange-rate provider (separate issue).

One thing I noticed while testing: the form lets you enable range mode and enter a fixed sats amount, which sends a fiat range + a single amountSats together the daemon rejects it as "Invalid amount." Independent of #282. Want me to file it as its own bug? PR: #302