[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/interface/widgets/general/layout_containers/flow_widget.cpp

@@ -7,6 +7,7 @@
 #include "flow_widget.h"
 
 #include <QHBoxLayout>
+#include <QKeyEvent>
 #include <QResizeEvent>
 #include <QScrollArea>
 #include <QSizePolicy>
@@ -177,6 +178,50 @@ QLayoutItem *FlowWidget::itemAt(int index) const
     return flowLayout->itemAt(index);
 }
 
+void FlowWidget::keyPressEvent(QKeyEvent *event)
+{
+    // Keyboard navigation between the flow items: arrow keys move focus just
+    // like clicking the sibling tiles would. Only items that can take keyboard
+    // focus (e.g. the deck-preview tiles in shared-deck links) are visited.
+    const bool moveForward = event->key() == Qt::Key_Right || event->key() == Qt::Key_Down;
+    const bool moveBackward = event->key() == Qt::Key_Left || event->key() == Qt::Key_Up;
+    if (!moveForward && !moveBackward) {
+        QWidget::keyPressEvent(event);
+        return;
+    }
+
+    QList<QWidget *> focusableItems;
+    for (int i = 0; i < flowLayout->count(); ++i) {
+        QWidget *item = flowLayout->itemAt(i)->widget();
+        if (item != nullptr && (item->focusPolicy() & Qt::TabFocus)) {
+            focusableItems.append(item);
+        }
+    }
+
+    if (focusableItems.isEmpty()) {
+        QWidget::keyPressEvent(event);
+        return;
+    }
+
+    int currentIndex = -1;
+    for (int i = 0; i < focusableItems.size(); ++i) {
+        if (focusableItems.at(i)->hasFocus()) {
+            currentIndex = i;
+            break;
+        }
+    }
+
+    const int delta = moveForward ? 1 : -1;
+    int nextIndex;
+    if (currentIndex < 0) {
+        nextIndex = moveForward ? 0 : focusableItems.size() - 1;
+    } else {
+        nextIndex = (currentIndex + delta + focusableItems.size()) % focusableItems.size();
+    }
+    focusableItems.value(nextIndex)->setFocus();
+    event->accept();
+}
+
 int FlowWidget::count() const
 {
     return flowLayout->count();

cockatrice/src/interface/widgets/general/layout_containers/flow_widget.h

@@ -11,6 +11,7 @@
 #include "../../../layouts/flow_layout.h"
 
 #include <QHBoxLayout>
+#include <QKeyEvent>
 #include <QLoggingCategory>
 #include <QScrollArea>
 #include <QWidget>
@@ -44,6 +45,7 @@ public slots:
 
 protected:
     void resizeEvent(QResizeEvent *event) override;
+    void keyPressEvent(QKeyEvent *event) override;
 
 private:
     Qt::Orientation flowDirection;