fix(balance): stop the displayed balance cratering during a pending shielded send

Sending a small amount from an address holding a large balance made the displayed
balance collapse to ~0 until the tx confirmed. A shielded spend consumes the whole
source note; the change returns as a 0-confirmation note, and every balance query
used the default minconf=1 — so the spent note dropped out and the change wasn't
counted yet.

Split every balance into two views:

- DISPLAY (balance / privateBalance / transparentBalance / totalBalance) — now
  queried at minconf=0, so it INCLUDES the user's own pending change and no longer
  craters. This is what the Overview, balance tab, market portfolio and receive
  tab show. unconfirmedBalance is now populated (= total - spendable).
- SPENDABLE (new spendableBalance / spendable*Balance) — confirmed (minconf>=1),
  what z_sendmany (run at minconf=1) can actually spend. The Send form's available/
  Max/validation, the from-address selection, the drag-to-transfer dialog cap, the
  chat pay-from and the auto-shield gate all size off these, so they never offer
  0-conf change the daemon would reject.

Implementation: a single z_listunspent(0)/listunspent(0), partitioned per-note by
"confirmations">=1; z_gettotalbalance called at minconf 0 (display) and 1
(spendable); the z_getbalance fallback queries both. applyPendingSendDelta (the
optimistic post-send debit) now touches ONLY the spendable fields — debiting the
display too would re-crater it on top of the honest minconf=0 RPC. Lite mirrors
spendableBalance = balance (its per-address balance is already confirmed) so lite
sends aren't zeroed. The confirmed-only gates (seed-migration/sweep z_gettotalbalance,
sweep z_getbalance(addr,1), z_sendmany's minconf arg) are untouched.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/data/wallet_state.cpp

@@ -19,12 +19,13 @@ std::vector<size_t> sortedSpendableAddressIndices(const std::vector<AddressInfo>
     for (size_t i = 0; i < addresses.size(); ++i) {
         const auto& address = addresses[i];
         if (!address.isSpendable()) continue;
-        if (requirePositiveBalance && address.balance <= 0.0) continue;
+        // Rank/filter by the CONFIRMED balance — an address holding only 0-conf change can't be sent from.
+        if (requirePositiveBalance && address.spendableBalance <= 0.0) continue;
         indices.push_back(i);
     }
 
     std::sort(indices.begin(), indices.end(), [&](size_t lhs, size_t rhs) {
-        return addresses[lhs].balance > addresses[rhs].balance;
+        return addresses[lhs].spendableBalance > addresses[rhs].spendableBalance;
     });
     return indices;
 }
@@ -34,8 +35,8 @@ int bestSpendableAddressIndex(const std::vector<AddressInfo>& addresses)
     int bestIndex = -1;
     double bestBalance = 0.0;
     for (size_t i = 0; i < addresses.size(); ++i) {
-        if (addresses[i].isSpendable() && addresses[i].balance > bestBalance) {
-            bestBalance = addresses[i].balance;
+        if (addresses[i].isSpendable() && addresses[i].spendableBalance > bestBalance) {
+            bestBalance = addresses[i].spendableBalance;
             bestIndex = static_cast<int>(i);
         }
     }