feat(app): suggest fee rates from the server and enforce the relay fee

After connecting, the Send tab offers Fast/Normal/Economy rates from the
server's fee estimates (never below the relay fee, implausible values
ignored). A manual rate below the relay fee is refused before building and
a fee above 1% of the amount is flagged in the preview. See ADR 0039.

SECURITY.md

@@ -35,6 +35,7 @@ The indexing server (ElectrumX-compatible, port 50001/50002) is a **semi-trusted
 - Lie about unconfirmed (mempool) transactions — the wallet shows mempool transactions as unconfirmed and non-spendable
 - Refuse to relay a broadcast transaction
 - Delay reporting of new blocks
+- Suggest misleading fee estimates or relay fee (the wallet ignores implausible values, never goes below the relay fee, and always shows the absolute fee, flagging one above 1% of the amount — ADR 0039)
 
 It cannot (given correct Merkle verification):