[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_alts.h

@@ -0,0 +1,54 @@
+#ifndef TAB_ALTS_H
+#define TAB_ALTS_H
+
+#include "tab.h"
+
+#include <QString>
+#include <QtGlobal>
+#include <qtmetamacros.h>
+
+class AbstractClient;
+class QLabel;
+class QLineEdit;
+class QPushButton;
+class QSpinBox;
+class QTableWidget;
+class Response;
+class TabSupervisor;
+
+/**
+ * Staff alt-graph investigation tool. Shows accounts related to the
+ * investigated user (shared email, client id, or recent IP address),
+ * including transitive alts-of-alts, with per-row match evidence.
+ */
+class TabAlts : public Tab
+{
+    Q_OBJECT
+public:
+    explicit TabAlts(TabSupervisor *_tabSupervisor, AbstractClient *_client, const QString &initialUser = {});
+    void retranslateUi() override;
+    [[nodiscard]] QString getTabText() const override
+    {
+        return tr("User Alts");
+    }
+    void investigate(const QString &userName);
+
+private slots:
+    void investigateUser();
+    void altsResponse(const Response &response);
+
+private:
+    void requestAlts(const QString &userName, int depth);
+    [[nodiscard]] QString formatEpoch(quint64 ts) const;
+
+    AbstractClient *client;
+    QString currentUser;
+
+    QLineEdit *searchEdit;
+    QSpinBox *depthSpin;
+    QPushButton *investigateButton;
+    QLabel *statusLabel;
+    QTableWidget *altsTable;
+};
+
+#endif // TAB_ALTS_H