refactor(config): replace SUPER_MODE_* macros with SuperMode enum

Type Config.super_mode as SuperMode (a proper C enum) instead of a bare
int.  The wire boundary still carries the mode as an int: send casts the
enum explicitly and receive reads a temporary int, validates the
AUTO..OFF range, then casts.  Emitted bytes and accepted values are
unchanged.  ModuleGateContext.super_mode_override keeps its -1 sentinel
as int with an explicit cast at the apply site.

Behavior preserved.

src/server/server.c

@@ -458,7 +458,7 @@ void handler(int file_descriptor) {
    * device-node creation) sees SUPER_MODE_OFF.  The gate never mutated the
    * received config. */
   if (gate_ctx.super_mode_override != -1)
-    config->super_mode = gate_ctx.super_mode_override;
+    config->super_mode = (SuperMode)gate_ctx.super_mode_override;
   protocol_set_8_bit_output(config->eight_bit_output);
   if (!authorized_root) {
     log_message(LOG_LEVEL_ERROR, "No server-side destination root configured");