[DeckShare] Address review findings and harden the share flows

Gate every share entry point on login, de-duplicate the share-link and
color-identity logic behind DeckShareUtils and an injected querier, and
replace the silent tray/status-bar notices with always-visible dialogs.

- abstract_tab_deck_editor: explain that sharing requires a connection
  instead of silently doing nothing when logged out
- tab_deck_storage: disable the share action on disconnect, reject
  folder/deck mixes and the root folder with clear warnings, re-enable
  Create on every entry/response so a dropped connection cannot leave
  the button disabled
- tab_deck_storage_visual: same login gate for the context-menu entry,
  visible success/error dialogs, and a symmetric in-flight guard
- getDeckColorIdentity now takes a CardDatabaseQuerier, dropping the
  CardDatabaseManager singleton access and enabling unit tests

cockatrice/src/interface/widgets/deck_share/share_bar_widget.h

@@ -42,6 +42,9 @@ public:
     /** @brief Sets the explainer hint text, showing it when @p visible is true. */
     void setHintText(const QString &text, bool visible);
 
+    /** @brief Enables or disables the create-share-link button (guards double submission). */
+    void setCreateEnabled(bool enabled);
+
     /** @brief Moves keyboard focus to the name field. */
     void focusName();