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.c

@@ -294,6 +294,14 @@ inp_get_line(void)
     return line;
 }
 
+void
+inp_set_line(const char* const new_line)
+{
+    rl_replace_line(new_line, 1);
+    rl_point = rl_end;
+    _inp_redisplay();
+}
+
 char*
 inp_get_password(void)
 {