[DeckEditor] Address zone-management review feedback

- Expose DecklistNodeTree::hasZoneName and use it in validateNewZoneName
  so the uniqueness scan covers custom zones on every board, not just the
  standard ones.
- Hide the board selector in the rename dialog path where it is not used.
- Emit deckHashChanged after refreshDeckHash so the deck hash label stays
  current after zone create/rename/move/remove.

libcockatrice_deck_list/libcockatrice/deck_list/deck_list_node_tree.h

@@ -115,6 +115,15 @@ public:
      */
     QList<const InnerDecklistNode *> getCustomZones(const QString &boardZoneName) const;
 
+    /**
+     * @brief Checks whether a zone name is taken anywhere in the deck.
+     *
+     * Covers the standard board names and any top-level or nested custom zone.
+     * @param zoneName The checked name.
+     * @return true if the name is reserved or already in use.
+     */
+    bool hasZoneName(const QString &zoneName) const;
+
     /**
      * @brief Applies a function to every card in the deck tree. This can modify the cards.
      *
@@ -129,7 +138,6 @@ private:
     InnerDecklistNode *findBoardZone(const QString &boardZoneName) const;
     InnerDecklistNode *findOrCreateBoardZone(const QString &boardZoneName);
     InnerDecklistNode *findCustomZoneByName(const QString &zoneName) const;
-    bool hasZoneName(const QString &zoneName) const;
 };
 
 #endif // COCKATRICE_DECKLIST_NODE_TREE_H