[Client] Transient User Alts investigation tab (#7511)

* [Client] Transient User Alts investigation tab

New TabAlts tab with user search, depth selector (1-3), and an alt table showing per-row match evidence (Via, Depth, Shared fan-out) alongside the account data. Sends Command_GetUserAlts with the requested depth and degrades gracefully against old servers (missing evidence fields render as dashes). Wired as a transient single-reuse tab via TabSupervisor::openTabAlts with no menu action or persisted setting; closed on disconnect like the other staff tabs.

* Fix remaining review threads on the user alts tab

- Switch to the User Alts tab when it is created, so the first investigate
  click does not open it in the background.
- Treat an empty RespOk as "user not found" and use a generic message for
  error codes, matching what the server actually returns.
- Exclude the depth-0 seed row from the related-account count.
- Guard the reply with the requested user name so a stale response cannot
  overwrite a newer one.
- Move the duplicated formatEpoch body into widgets/utility/epoch_format.h.

---------

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

cockatrice/src/interface/widgets/tabs/tab_supervisor.h

@@ -50,6 +50,7 @@ class TabAdmin;
 class TabMessage;
 class TabReport;
 class TabModeration;
+class TabAlts;
 class TabAccount;
 class TabDeckEditor;
 class TabDeveloper;
@@ -116,6 +117,7 @@ private:
     TabLog *tabLog;
     TabReport *tabReport;
     TabModeration *tabModeration;
+    TabAlts *tabAlts;
     TabDeveloper *tabDeveloper;
     QMap<int, TabRoom *> roomTabs;
     QMap<int, TabGame *> gameTabs;
@@ -209,6 +211,7 @@ public slots:
     /** Joins or spectates a game listed on a user's profile hover card. */
     void joinGameFromUserCard(int gameId, int roomId, bool asSpectator);
     void openTabModeration(const QString &userName = {});
+    void openTabAlts(const QString &userName = {});
     void switchToFirstAvailableNetworkTab();
     void maximizeMainWindow();
     void actTabVisualDeckStorage(bool checked);