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/inputwin.h

@@ -46,5 +46,6 @@ void inp_win_resize(void);
 void inp_put_back(void);
 char* inp_get_password(void);
 char* inp_get_line(void);
+void inp_set_line(const char* const new_line);
 
 #endif