[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.cpp

@@ -67,6 +67,11 @@ void ShareBarWidget::setHintText(const QString &text, bool visible)
     hintLabel->setVisible(visible);
 }
 
+void ShareBarWidget::setCreateEnabled(bool enabled)
+{
+    createButton->setEnabled(enabled);
+}
+
 void ShareBarWidget::focusName()
 {
     nameEdit->setFocus();