Always keep a change output, so every tx stays fee-bumpable (B-62)

The max-amount checkbox sends amount_sats == the whole confirmed balance, so
change came out at 0, the change output was dropped, and the transaction had a
single output. bump_fee has nothing to shrink there: it raised RbfError every
30s until the reconciler abandoned the row six hours later. The RBF
single-change-output limitation was a documented gap, but the UI made it the
*default* withdrawal path.

The extra-input fallback would not have helped this case: a transaction moving
the entire balance already spends every UTXO the sender has. So the fix is at
build time — build_signed_transaction never produces a change output below
DUST_LIMIT_SATS, and never folds it into the fee either:

- withdrawals pass reduce_amount_to_keep_change=True and move a dust limit less.
  The fee already comes out of the withdrawn amount by design, so this is the
  same rule applied a little harder, and Withdrawal.amount_requested_sats vs
  amount_sent_sats already existed to record the difference.
- bets don't: the bet is a fixed price that can't be quietly reduced. A balance
  exactly equal to the bet is refused with balance_leaves_no_change (translated
  into all 7 languages, carrying required_extra_sats), which turns "a user's
  balance must never exactly equal the bet" from a documented assumption into an
  enforced one — and stops an unbumpable bet from holding a round open until the
  reconciler gives up on it.

bump_fee's no-change guard stays: a single-output tx broadcast before this
change can still be pending across the deploy, and it must fail loudly rather
than start shrinking a recipient's output. Its test now hand-builds that shape,
precisely because the builder no longer will.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/guida-utente.md

@@ -128,6 +128,12 @@ scalata dall'importo richiesto (non si aggiunge separatamente). L'importo
 minimo prelevabile è pari alla quota fissa di ingresso al round (mostrata
 nella sezione Bet).
 
+Con "Preleva l'intero importo" restano sul tuo saldo pochi satoshi (294, cioè
+0,00000294 PLM): senza quel resto la transazione non potrebbe essere
+ritrasmessa con una fee più alta se la rete fosse lenta, e resterebbe bloccata
+per ore. La cifra effettivamente inviata è quindi il saldo meno quei satoshi e
+meno la fee di rete.
+
 > **Nota**: attualmente è supportato solo l'indirizzo esterno in formato
 > **P2WPKH bech32** (quelli che iniziano con `plm1q...`). Non inserire
 > indirizzi legacy (quelli che iniziano con `P...`) o P2SH: al momento