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/transaction_details_dialog.cpp

@@ -179,10 +179,15 @@ void TransactionDetailsDialog::render(App* app)
         float start_x = (ImGui::GetWindowWidth() - total_width) / 2.0f;
         ImGui::SetCursorPosX(start_x);
         
+        // Guard against an empty/whitespace explorer URL so we never open a garbage link.
+        std::string explorerBase = app->settings()->getTxExplorerUrl();
+        bool explorerValid = explorerBase.find_first_not_of(" \t\r\n") != std::string::npos;
+        if (!explorerValid) ImGui::BeginDisabled();
         if (material::StyledButton(TR("tx_view_explorer"), ImVec2(button_width, 0), S.resolveFont(bottomBtn.font))) {
-            std::string url = app->settings()->getTxExplorerUrl() + tx.txid;
+            std::string url = explorerBase + tx.txid;
             util::Platform::openUrl(url);
         }
+        if (!explorerValid) ImGui::EndDisabled();
         
         ImGui::SameLine();