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/address_label_dialog.h

@@ -221,7 +221,19 @@ public:
             ImGui::SameLine(0, Layout::spacingMd());
             if (TactileButton(saveLabel, ImVec2(saveW, 0), buttonFont)) {
                 // Apply changes
-                s_app->setAddressLabel(s_address, s_label);
+                // Trim surrounding whitespace; a whitespace-only label clears it
+                // (mirroring how an empty icon selection clears the icon).
+                std::string trimmedLabel(s_label);
+                {
+                    std::size_t b = trimmedLabel.find_first_not_of(" \t\r\n");
+                    if (b == std::string::npos) {
+                        trimmedLabel.clear();
+                    } else {
+                        std::size_t e = trimmedLabel.find_last_not_of(" \t\r\n");
+                        trimmedLabel = trimmedLabel.substr(b, e - b + 1);
+                    }
+                }
+                s_app->setAddressLabel(s_address, trimmedLabel);
                 if (s_selectedIcon >= 0)
                     s_app->setAddressIcon(s_address, material::project_icons::walletIconName(s_selectedIcon));
                 else