feat(bootstrap,validate): Retry keeps the mirror; Validate works offline

- Bootstrap: Retry now reuses the source the user actually started (was always
  the default), and a "Try mirror" button offers the alternate server on
  failure (s_lastUrl remembered on start, cleared on a fresh Confirm).
- Validate Address: classification + checksum are local, so the dialog no
  longer requires a node connection — offline/lite now validate and show the
  type, with ownership reported as "Unknown — node offline / not checkable in
  the lite wallet" instead of a dead "not connected" screen.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ui/windows/bootstrap_download_dialog.h

@@ -45,6 +45,7 @@ public:
         s_state = State::Confirm;
         s_bootstrap.reset();
         s_errorMsg.clear();
+        s_lastUrl.clear();
         s_wasDaemonRunning = false;
     }
 
@@ -314,9 +315,15 @@ private:
 
         float btnW = 120.0f * dp;
         if (TactileButton(TR("retry"), ImVec2(btnW, 0))) {
-            startDownload("");
+            startDownload(s_lastUrl);   // keep the user's source/mirror choice (was startDownload(""))
         }
         ImGui::SameLine();
+        {
+            const std::string mirrorUrl = std::string(util::Bootstrap::kMirrorUrl) + "/" + util::Bootstrap::kZipName;
+            if (s_lastUrl != mirrorUrl && TactileButton(TR("bootstrap_try_mirror"), ImVec2(130.0f * dp, 0)))
+                startDownload(mirrorUrl);
+            ImGui::SameLine();
+        }
         if (TactileButton(TR("close"), ImVec2(90.0f * dp, 0))) {
             s_app->setBootstrapDownloading(false);
             s_open = false;
@@ -336,6 +343,7 @@ private:
             s_bootstrap->start(dataDir);
         else
             s_bootstrap->start(dataDir, url);
+        s_lastUrl = url;   // remember what we started (set only after the guards passed)
         s_state = State::Downloading;
         s_errorMsg.clear();
         s_cancelling = false;
@@ -347,6 +355,7 @@ private:
     static inline std::unique_ptr<util::Bootstrap> s_bootstrap;
     static inline bool s_wasDaemonRunning = false;
     static inline bool s_cancelling = false;  // cancel requested; disables the Cancel button until the worker stops
+    static inline std::string s_lastUrl;      // last source started ("" = default); so Retry keeps the mirror choice
     static inline std::string s_errorMsg;
 };