[UserList] Context menu invite (#7138)

* [Client] Send game invites from the user context menu via a private message

The user context menu gains an "Invite to Game" submenu listing the
inviteable games in the room (the inviter's own games, honoring the
buddy-only setting). Picking one opens a private message to the target
user with a cockatrice://joingame link naming the game, so the target
gets a clickable invite instead of a raw URL. Multi-game rooms offer a
picker; a single inviteable game sends directly. Sending a message to
an offline user no longer swallows the draft — it reports that the
user is offline and keeps the typed text.

Took 50 seconds


Took 3 minutes

* [Client] Extract sendPrivateMessage() to fix invite message draft overwrite

sendInviteMessage() was calling sayEdit->setText(text) then sendMessage(),
which overwrites any text the user had typed.  Extract the command-building
and sending logic into a new sendPrivateMessage(const QString &text) method
that takes the text directly.  sendMessage() now calls it after its guards
and clears sayEdit; sendInviteMessage() calls it directly without touching
the input field at all.

Took 33 minutes

* Rename method, address comments.

Took 5 minutes

---------

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

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

@@ -64,6 +64,9 @@ public:
     void processUserLeft();
     void processUserJoined(const ServerInfo_User &_userInfo);
 
+    [[nodiscard]] bool isUserOnline() const;
+    void sendPrivateMessage(const QString &text);
+
 private:
     bool shouldShowSystemPopup(const Event_UserMessage &event);
     void showSystemPopup(const Event_UserMessage &event);