feat: Add /force-encryption command and update message handling

Implement /force-encryption command to configure forced encryption
settings with on|off and policy resend-to-confirm|block options.

The changes enhance user experience by clarifying encryption requirements
and providing actionable commands. Users can now press Enter again to
confirm unencrypted messages in resend-to-confirm mode.

src/ui/window.c

@@ -164,6 +164,7 @@ win_create_chat(const char* const barejid)
     new_win->last_message = NULL;
     new_win->last_msg_id = NULL;
     new_win->has_attention = FALSE;
+    new_win->last_attempted_msg_hash = 0;
     new_win->memcheck = PROFCHATWIN_MEMCHECK;
 
     return &new_win->window;