[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_moderation.cpp

@@ -1,5 +1,6 @@
 #include "tab_moderation.h"
 
+#include "../utility/epoch_format.h"
 #include "abstract_client.h"
 #include "libcockatrice/protocol/pb/admin_commands.pb.h"
 #include "libcockatrice/protocol/pb/response.pb.h"
@@ -215,10 +216,7 @@ void TabModeration::retranslateUi()
 
 QString TabModeration::formatEpoch(quint64 ts) const
 {
-    if (ts == 0) {
-        return tr("Unknown");
-    }
-    return QDateTime::fromSecsSinceEpoch(ts).toLocalTime().toString("yyyy-MM-dd HH:mm");
+    return formatEpochTimestamp(ts, tr("Unknown"));
 }
 
 void TabModeration::clearUserData()