[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/intents/intent.cpp

@@ -2,10 +2,11 @@
 
 Intent::Intent(QObject *parent) : QObject(parent)
 {
-    // An intent is done as soon as it reports success or failure. Deleting it
-    // also tears down its dependency chain and disconnects any signal wiring.
+    // An intent is done as soon as it reports success, failure, or cancellation.
+    // Deleting it also tears down its dependency chain and disconnects any signal wiring.
     connect(this, &Intent::finished, this, &QObject::deleteLater);
     connect(this, &Intent::failed, this, &QObject::deleteLater);
+    connect(this, &Intent::cancelled, this, &QObject::deleteLater);
 }
 
 Intent::~Intent() = default;
@@ -46,3 +47,11 @@ void Intent::emitFailed(const QString &reason)
         emit failed(reason);
     }
 }
+
+void Intent::emitCancelled()
+{
+    if (!completed) {
+        completed = true;
+        emit cancelled();
+    }
+}