[DeckShare] Open shared decks via links with a gated preview flow

- Serialized url-chain dispatcher in IntentUrlParser; queue-drained
  urlChainFinished(bool) drives the startup auto-connect fallback
- Open-shared-deck intent with sequential download state machine,
  15s per-item timeout, partial-success offer, livable Cancel via
  ApplicationModal dlg_login_prompt interactive fallback
- Preview dialog: download progress label, share vocab sweep,
  palette-highlight selection frame, Space/Enter keyboard toggle,
  NoFocus checkbox, double-click tile opens immediately
- Confirm-before-server-migration with one-shot restore to the
  previous server on failed/cancelled chains (statusChanged settle
  deferral), hostname-only identity comparisons
- Skip credential link when already connected; arrow-key navigation
  in FlowWidget; card glows use palette highlight
- Address code-review M1-M4 and UI/UX QA blockers 1-2

cockatrice/src/single_instance_manager.cpp

@@ -2,6 +2,14 @@
 
 #include <QDir>
 
+namespace
+{
+// Sent by the primary instance after it has read a forwarded payload. Without
+// an acknowledgment, a second instance cannot tell a live primary apart from a
+// stale socket left behind by a process that is still shutting down.
+const QByteArray ACK_MESSAGE = QByteArrayLiteral("COCKATRICE_ACK");
+} // namespace
+
 SingleInstanceManager::SingleInstanceManager(QObject *parent) : QObject(parent)
 {
 }
@@ -72,7 +80,14 @@ bool SingleInstanceManager::forwardToPrimary(const QStringList &filesToSend)
     socket.flush();
     socket.waitForBytesWritten(1000);
 
-    return true;
+    // Only report a successful hand-off once the primary has acknowledged that
+    // it actually read the payload. A socket that connects but never answers
+    // belongs to a process that is dying, so the caller must not treat this as
+    // a hand-off (otherwise it would exit without anyone handling the files).
+    if (!socket.waitForReadyRead(1000)) {
+        return false;
+    }
+    return socket.readAll() == ACK_MESSAGE;
 }
 
 void SingleInstanceManager::handleNewConnection()
@@ -111,6 +126,14 @@ void SingleInstanceManager::handleNewConnection()
             QStringList files;
             payloadStream >> files;
 
+            // Acknowledge receipt as soon as the payload is parsed, before the
+            // primary starts handling it. The handlers run synchronously and can
+            // take longer than the sender's readiness timeout (e.g. a modal
+            // confirmation box), which would otherwise make a live primary look
+            // dead and cause duplicate handling.
+            socket->write(ACK_MESSAGE);
+            socket->flush();
+
             emit filesReceived(files);
 
             // Reset buffer (single message use-case)