i18n: route explorer and shield status strings through TR()

Centralise the remaining hardcoded English literals in two otherwise
fully-translated files (explorer_tab: 40 TR calls, shield_dialog: 23) so
they join the i18n system with an English source key + fallback:

- explorer_tab: the three search-error messages (invalid response, hash
  not found, not-connected) now use explorer_* keys. Also guards a hash
  lookup behind an active daemon connection and disables the block-detail
  prev/next nav while a fetch is in flight (both were the reason these
  error paths could fire).
- shield_dialog: the operation status/error strings (submitting, submitted,
  failed, status, error-checking-status, shield/merge failed) now use
  shield_* keys.
- i18n: add the new explorer_* and shield_* English source keys.

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

src/ui/windows/explorer_tab.cpp

@@ -221,7 +221,7 @@ static void applyBlockDetailResult(const json& result, const std::string& error)
     }
 
     if (result.is_null()) {
-        s_search_error = "Invalid response from daemon";
+        s_search_error = TR("explorer_invalid_response");
         s_show_detail_modal = false;
         return;
     }
@@ -388,7 +388,7 @@ static void navigateToHash(App* app, const std::string& hash) {
                 s_expanded_tx_idx = 0;
                 s_show_detail_modal = false;
             } else {
-                s_search_error = "No block or transaction found for this hash";
+                s_search_error = TR("explorer_hash_not_found");
             }
         };
     });
@@ -417,6 +417,11 @@ static void performSearch(App* app, const std::string& query) {
             return (c >= '0' && c <= '9') || (c >= 'a' && c <= 'f') || (c >= 'A' && c <= 'F');
         });
     if (isHex64) {
+        // A hash lookup needs the daemon; navigateToHash() would otherwise return silently.
+        if (!app->rpc() || !app->rpc()->isConnected()) {
+            s_search_error = TR("explorer_not_connected");
+            return;
+        }
         navigateToHash(app, query);
         return;
     }
@@ -1039,6 +1044,9 @@ static void renderBlockDetailModal(App* app) {
         // Nav buttons on same line
         ImGui::SameLine(contentW - Layout::spacingXl() * 3);
 
+        // Disable prev/next while a fetch is in flight so navigation can't stack fetches.
+        ImGui::BeginDisabled(s_detail_loading);
+
         // Prev
         if (s_detail_height > 1) {
             ImGui::PushFont(Type().iconMed());
@@ -1065,6 +1073,8 @@ static void renderBlockDetailModal(App* app) {
             ImGui::PopID();
             ImGui::PopFont();
         }
+
+        ImGui::EndDisabled();
     }
 
     ImGui::Separator();