Studio: reset vision_image_size in the model-config error fallback path

mapBackendModelConfigToTrainingPatch resets stale image size on the
success path, but if the /api/models/config endpoint throws,
training-config-store.ts falls through to checkVisionModel and only
updates capability flags. Pre-fix that left a stale 2048 (or any
prior selection) in the store, so once dataset detection marked the
new dataset as image, the next training start would silently apply
the previous model's size. The error branch now also resets to the
DEFAULT_HYPERPARAMS.visionImageSize sentinel.

studio/frontend/src/features/training/stores/training-config-store.ts

@@ -390,6 +390,10 @@ export const useTrainingConfigStore = create<TrainingConfigStore>()(
                 error instanceof Error
                   ? error.message
                   : "Failed to load model defaults",
+              // Defaults load failed, so we cannot map the new model's
+              // vision_image_size. Reset to the global default so a stale
+              // value from a prior model never silently applies.
+              visionImageSize: DEFAULT_HYPERPARAMS.visionImageSize,
             });
 
             // Fallback vision check if config endpoint fails.