Studio: align YAML export gate with API mapper + extend Image Size dropdown

- training-section.tsx: handleSaveConfig now passes
  isVisionModel && isDatasetImage === true to serializeConfigToYaml,
  matching buildTrainingStartPayload. Stops vision_image_size from
  leaking into exported YAML for text-only datasets where the API
  would have sent null.
- params-section.tsx: add 256 to visionImageSizePresets so the
  dropdown spans the validator's full [256, 2048] range. Also render
  a synthetic SelectItem for the current value when it was loaded
  from YAML or model defaults and is not in the preset list, so the
  controlled Select always shows the active size.

studio/frontend/src/features/studio/sections/params-section.tsx

@@ -139,7 +139,8 @@ export function ParamsSection(): ReactElement {
   const [ctxInput, setCtxInput] = useState(String(store.contextLength));
   const ctxAnchorRef = useRef<HTMLDivElement>(null);
   const ctxItems = CONTEXT_LENGTHS.map(String);
-  const visionImageSizePresets = [384, 512, 768, 1024, 1536, 2048];
+  // Backend validator allows [256, 2048]; offer the full span.
+  const visionImageSizePresets = [256, 384, 512, 768, 1024, 1536, 2048];
 
   // Keep input in sync when the store value changes externally
   // (e.g. model defaults being applied after model selection).
@@ -953,6 +954,16 @@ export function ParamsSection(): ReactElement {
                         </SelectTrigger>
                         <SelectContent>
                           <SelectItem value="default">Default</SelectItem>
+                          {store.visionImageSize != null &&
+                            !visionImageSizePresets.includes(
+                              store.visionImageSize,
+                            ) && (
+                              <SelectItem
+                                value={String(store.visionImageSize)}
+                              >
+                                {store.visionImageSize}
+                              </SelectItem>
+                            )}
                           {visionImageSizePresets.map((size) => (
                             <SelectItem key={size} value={String(size)}>
                               {size}

studio/frontend/src/features/studio/sections/training-section.tsx

@@ -80,7 +80,11 @@ export function TrainingSection() {
   };
 
   const handleSaveConfig = () => {
-    const yamlStr = serializeConfigToYaml(store, store.isVisionModel);
+    // Match the API mapper gate so exported YAML never carries
+    // vision_image_size for non-image datasets.
+    const includeVisionFields =
+      store.isVisionModel && store.isDatasetImage === true;
+    const yamlStr = serializeConfigToYaml(store, includeVisionFields);
     const blob = new Blob([yamlStr], { type: "text/yaml" });
     const url = URL.createObjectURL(blob);
     const a = document.createElement("a");