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/app_sweep.cpp

@@ -170,10 +170,10 @@ void App::installDemoWalletData()
     }
 
     auto zaddr = [](const char* a, double bal, const char* label) {
-        AddressInfo i; i.address = a; i.balance = bal; i.type = "shielded"; i.label = label; return i;
+        AddressInfo i; i.address = a; i.balance = bal; i.spendableBalance = bal; i.type = "shielded"; i.label = label; return i;
     };
     auto taddr = [](const char* a, double bal, const char* label) {
-        AddressInfo i; i.address = a; i.balance = bal; i.type = "transparent"; i.label = label; return i;
+        AddressInfo i; i.address = a; i.balance = bal; i.spendableBalance = bal; i.type = "transparent"; i.label = label; return i;
     };
     state_.z_addresses = {
         zaddr("zs1demoprimaryshieldedaddressforuisweep000000000000000000000000000", 12.0, "Savings"),