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/ui/windows/send_tab.cpp

@@ -246,15 +246,16 @@ static void RenderSourceDropdown(App* app, float width) {
     Type().textColored(TypeStyle::Overline, OnSurfaceMedium(), TR("send_sending_from"));
     ImGui::Dummy(ImVec2(0, Layout::spacingSm()));
 
-    // Auto-select the address with the largest balance on first load
-    if (!s_auto_selected && app->isConnected() && !state.addresses.empty()) {
-        int bestIdx = bestSpendableAddressIndex(state.addresses);
-        if (bestIdx >= 0) {
-            s_selected_from_idx = bestIdx;
-            snprintf(s_from_address, sizeof(s_from_address), "%s",
-                     state.addresses[bestIdx].address.c_str());
-        }
-        s_auto_selected = true;
+    // s_from_address is the single source of truth (it is what gets debited). Auto-select the address with the
+    // largest balance on first load ONLY when no source is set yet - never over a "Send from this address"
+    // pre-fill from another tab - and re-derive the list index from the string every frame so it can't desync
+    // when the address list changes.
+    {
+        const auto sel = resolveSourceSelection(state.addresses, s_from_address, s_auto_selected, app->isConnected());
+        if (sel.address != s_from_address)
+            snprintf(s_from_address, sizeof(s_from_address), "%s", sel.address.c_str());
+        s_auto_selected = sel.autoLatched;
+        s_selected_from_idx = sel.index;
     }
 
     // Build preview string for selected address
@@ -271,6 +272,11 @@ static void RenderSourceDropdown(App* app, float width) {
                  tag, trunc.c_str(),
                  FormatSourceBalance(addr.balance, addr.spendableBalance).c_str());
         s_source_preview = buf;
+    } else if (s_from_address[0] != '\0') {
+        // A pre-filled source that the dropdown list doesn't carry (zero-balance / view-only / list still
+        // loading): show the address that WILL be debited rather than the "Select a source" placeholder.
+        s_source_preview = util::truncateMiddle(s_from_address,
+            static_cast<int>(std::max(S.drawElement("tabs.send", "addr-preview-trunc-min").size, width / S.drawElement("tabs.send", "addr-preview-trunc-divisor").size)));
     } else {
         s_source_preview = TR("send_select_source");
     }
@@ -1696,6 +1702,7 @@ void SetSendFromAddress(const std::string& address)
     strncpy(s_from_address, address.c_str(), sizeof(s_from_address) - 1);
     s_from_address[sizeof(s_from_address) - 1] = '\0';
     s_selected_from_idx = -1;
+    s_auto_selected = true;   // an explicit choice: the first-visit auto-pick must never replace it
 }
 
 } // namespace ui