[Game] Add an invite button to non-started and not full games (#7143)

* [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 1 minute

* [Client] Add invite-to-game dialog to the game window

Took 15 seconds

* [Client] Open the invite dialog taller by default without enforcing a minimum size

* Move button to bottom

Took 3 minutes

* Address comments.

---------

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

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

@@ -128,6 +128,12 @@ bool TabMessage::isUserOnline() const
     return userOnline;
 }
 
+void TabMessage::sendInviteMessage(const QString &text)
+{
+    sayEdit->setText(text);
+    sendMessage();
+}
+
 void TabMessage::messageSent(const Response &response,
                              const CommandContainer & /*commandContainer*/,
                              const QVariant &extraData)