[GameMode] Don't probe libgamemode on every config load, preserve user setting

Two related fixes for the GameMode startup behavior on Linux:

1. configuration.c: Only call frontend_driver_set_gamemode() during config
   load when gamemode_enable is actually true. Previously the probe ran
   unconditionally with the current setting value, meaning every user on
   every config load triggered a dlopen of libgamemode.so even when the
   feature was off - producing log spam ("[GameMode] GameMode cannot be
   enabled on this system...") for the majority of users who don't have
   libgamemode installed.

   Also stop force-disabling gamemode_enable in the config when the probe
   fails. Silently flipping the user's preference to false and persisting
   it to retroarch.cfg meant a transient failure (sandbox path issue,
   library not yet installed, multi-arch mismatch, etc.) would require
   the user to manually re-enable the option on every subsequent launch.
   The user's intent should survive across sessions; if libgamemode shows
   up later, the next launch picks it up automatically.

2. frontend/drivers/platform_unix.c: Latch the unavailable state inside
   frontend_unix_set_gamemode() once gamemode_query_status() reports
   failure. The result will not change for the lifetime of the process,
   so subsequent calls (deinit cleanup, menu toggles) short-circuit
   without re-entering the Feral client code and without re-emitting the
   warning. The warning now fires exactly once per process when the user
   has gamemode enabled but libgamemode is missing.

The redundant "[Config] GameMode unsupported - disabling..." follow-up
warning is removed - it added no information beyond what the GameMode
warning already conveys.

configuration.c

@@ -4653,14 +4653,16 @@ static bool config_load_file(global_t *global,
    if (!(bool)RHMAP_HAS_STR(conf->entries_map, "user_language"))
       msg_hash_set_uint(MSG_HASH_USER_LANGUAGE, frontend_driver_get_user_language());
 
-   if (frontend_driver_has_gamemode() &&
-         !frontend_driver_set_gamemode(settings->bools.gamemode_enable) &&
-         settings->bools.gamemode_enable)
-   {
-      RARCH_WARN("[Config] GameMode unsupported - disabling...\n");
-      configuration_set_bool(settings,
-            settings->bools.gamemode_enable, false);
-   }
+   /* If GameMode is enabled in the config but libgamemode is not
+    * available, warn once. Do NOT clear the setting: that would
+    * silently overwrite the user's preference in retroarch.cfg, so
+    * a transient failure (sandbox path issue, library not yet
+    * installed, etc.) would force them to re-enable it manually on
+    * every subsequent launch. The probe is latched inside the
+    * frontend driver, so it will not be retried this session. */
+   if (     settings->bools.gamemode_enable
+         && frontend_driver_has_gamemode())
+      frontend_driver_set_gamemode(true);
 
    /* If this is the first run of an existing installation
     * after the independent favourites playlist size limit was

frontend/drivers/platform_unix.c

@@ -2186,8 +2186,20 @@ static void android_app_destroy(struct android_app *android_app)
 static bool frontend_unix_set_gamemode(bool on)
 {
 #ifdef FERAL_GAMEMODE
-   int gamemode_status  = gamemode_query_status();
-   bool gamemode_active = (gamemode_status == 2);
+   /* Once gamemode_query_status() reports failure (typically because
+    * libgamemode.so is not installed), there is no point repeatedly
+    * re-probing on every config load or menu toggle - the result will
+    * not change for the lifetime of the process, and each probe emits
+    * a warning. Latch the unavailable state and short-circuit. */
+   static bool gamemode_unavailable = false;
+   int gamemode_status;
+   bool gamemode_active;
+
+   if (gamemode_unavailable)
+      return false;
+
+   gamemode_status  = gamemode_query_status();
+   gamemode_active  = (gamemode_status == 2);
 
    if (gamemode_status < 0)
    {
@@ -2196,6 +2208,7 @@ static bool frontend_unix_set_gamemode(bool on)
                "https://github.com/FeralInteractive/gamemode needs to be installed.\n",
                gamemode_error_string());
 
+      gamemode_unavailable = true;
       return false;
    }