fix(ui): keep a chosen send-from address from auto-select overwrite

The Send tab auto-selected the highest-balance address on first load.
When another tab had already pre-filled the source with "Send from this
address", that first-load pick overwrote the user's choice, and the
dropdown's selected index could drift out of step with the address that
would actually be debited when the address list changed.

- The send-from string is now the single source of truth. The one-shot
  auto-pick only fills an empty source and never replaces one the user
  or another tab set, and the list index is re-derived from the string
  each frame so it cannot desync.
- An explicit "Send from this address" choice latches the auto-pick off,
  so the first visit cannot undo it.
- A chosen source the dropdown does not list (zero balance, view-only,
  or the list still loading) now previews the address that will be
  debited instead of the "Select a source" placeholder.

User-visible: a send-from address chosen from another tab survives the
first load and later refresh ticks.

Tests: the source resolver keeps a pre-fill on first visit, fills an
empty source with the best spendable address, keeps a source missing
from a short list with index -1, latches nothing while disconnected or
with an empty list, and re-derives a stale index from the string.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/data/wallet_state.cpp

@@ -43,6 +43,32 @@ int bestSpendableAddressIndex(const std::vector<AddressInfo>& addresses)
     return bestIndex;
 }
 
+int addressIndexByString(const std::vector<AddressInfo>& addresses, const std::string& address)
+{
+    if (address.empty()) return -1;
+    for (size_t i = 0; i < addresses.size(); ++i) {
+        if (addresses[i].address == address) return static_cast<int>(i);
+    }
+    return -1;
+}
+
+SourceSelection resolveSourceSelection(const std::vector<AddressInfo>& addresses, const std::string& current,
+                                       bool autoLatched, bool connected)
+{
+    SourceSelection r;
+    r.address = current;
+    r.autoLatched = autoLatched;
+    r.index = addressIndexByString(addresses, current);
+    if (!autoLatched && connected && !addresses.empty()) {
+        if (current.empty()) {                       // only ever fill an empty source
+            const int best = bestSpendableAddressIndex(addresses);
+            if (best >= 0) { r.address = addresses[best].address; r.index = best; }
+        }
+        r.autoLatched = true;                        // one shot, whether or not it picked
+    }
+    return r;
+}
+
 std::string TransactionInfo::getTimeString() const
 {
     if (timestamp == 0) return "Unknown";

src/data/wallet_state.h

@@ -42,6 +42,20 @@ struct AddressInfo {
 std::vector<size_t> sortedSpendableAddressIndices(const std::vector<AddressInfo>& addresses,
                                                   bool requirePositiveBalance = true);
 int bestSpendableAddressIndex(const std::vector<AddressInfo>& addresses);
+// Index of the address whose string equals `address` (exact match), or -1 (empty string / not listed).
+int addressIndexByString(const std::vector<AddressInfo>& addresses, const std::string& address);
+
+// The Send tab's source-address resolution, pure. `current` (s_from_address) is the single source of truth:
+// the one-shot auto-pick (highest confirmed balance) only fills an EMPTY source, and never replaces one the
+// user - or another tab's "Send from this address" - already set. A source that is not in the list (the list
+// can be transiently short) is kept as is with index -1; it must NOT trigger a re-pick.
+struct SourceSelection {
+    std::string address;         // what s_from_address must hold afterwards
+    int         index = -1;      // its row in `addresses`, -1 if not listed
+    bool        autoLatched = false;   // new value of the one-shot latch
+};
+SourceSelection resolveSourceSelection(const std::vector<AddressInfo>& addresses, const std::string& current,
+                                       bool autoLatched, bool connected);
 
 /**
  * @brief Represents a wallet transaction