fix(ui): validate and clamp dialog input

Tighten input handling across several dialogs so bad/edge input can't
produce a doomed request or a confusing display:

- app_security: passphrase strength meter now factors character-class
  diversity — an all-one-class string downgrades one tier so "aaaaaaaa"
  no longer scores as high as a mixed one.
- block_info: clamp the height field to the chain tip and hide/deny
  "Next" at the tip (was only yielding a raw RPC error).
- address_label: trim surrounding whitespace before saving; a
  whitespace-only label clears it (mirrors clearing the icon).
- send_tab: re-clamp the amount when a Max send has its fee bumped in the
  confirm popup (kept the total within budget) and NUL-terminate the
  strncpy'd address/memo when a payment URI overwrites the form.
- settings_page: reject an empty/whitespace-only lite wallet path before
  dispatching an unusable open/restore request.
- peers_tab: guard ExtractIP against an empty address.
- transaction_details: disable "View in explorer" when the configured
  explorer URL is empty/whitespace so we never open a garbage link.
- app: seed-backup "Skip" now requires a second, deliberate click with a
  fund-loss warning before it dismisses.

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

src/ui/windows/send_tab.cpp

@@ -725,7 +725,17 @@ void RenderSendConfirmPopup(App* app) {
         {
             Type().textColored(TypeStyle::Overline, OnSurfaceMedium(), TR("send_network_fee"));
             ImGui::Dummy(ImVec2(0, Layout::spacingSm()));
+            double feeBefore = s_fee;
             RenderFeeTierSelector("##confirm");
+            // If Max was selected, a fee bump here would push total over the available
+            // balance (s_amount was clamped to available - old fee). Re-clamp so the send
+            // stays within budget.
+            if (s_send_max && s_fee != feeBefore) {
+                double avail = GetAvailableBalance(app);
+                double maxAmount = avail - s_fee;
+                if (maxAmount < 0) maxAmount = 0;
+                s_amount = maxAmount;
+            }
             // Recalculate total after potential fee change
             total = s_amount + s_fee;
             ImGui::Dummy(ImVec2(0, Layout::spacingMd()));
@@ -1129,9 +1139,16 @@ void RenderSendTab(App* app)
 
     // Handle pending payment from URI
     if (app->hasPendingPayment()) {
+        // A URI arriving mid-compose would silently overwrite the user's in-progress form.
+        // Capture an undo snapshot first so "Undo" can restore what they were typing.
+        if (FormHasData()) {
+            SaveFormSnapshot();
+        }
         strncpy(s_to_address, app->getPendingToAddress().c_str(), sizeof(s_to_address) - 1);
+        s_to_address[sizeof(s_to_address) - 1] = '\0';
         s_amount = app->getPendingAmount();
         strncpy(s_memo, app->getPendingMemo().c_str(), sizeof(s_memo) - 1);
+        s_memo[sizeof(s_memo) - 1] = '\0';
         app->clearPendingPayment();
     }