[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/tabs/tab_deck_storage.h

@@ -45,7 +45,7 @@ private:
 
     void setRemoteEnabled(bool enabled);
 
-    void showShareNotice(const QString &message);
+    void showShareNotice(const QString &message, bool warning = false);
 
     void setShareModeEnabled(bool enabled);