Prevent image editing without image input (#1410)

## Motivation

Models that only support image editing (ImageToImage but not
TextToImage) would silently attempt text-to-image generation when a user
submitted a text prompt without an attached image

## Changes

- Added an early return guard in handleSubmit() that prevents submission
when the selected model only supports image editing and no image is
attached (isEditOnlyWithoutImage)
- Fixed the text-to-image generation branch to use the more specific
modelSupportsTextToImage() check instead of the broad isImageModel(),
ensuring only models with TextToImage capability trigger generation from
text alone
- The existing isEditOnlyWithoutImage derived state (which was already
used for UI hints like placeholder text and button disabling) now also
blocks the actual submit path

## Why It Works

The text-to-image fallback now correctly checks
modelSupportsTextToImage() directly, so edit-only models no longer fall
through to the generation path


## Test Plan

### Manual Testing

- Select an edit-only image model (e.g., one with only ImageToImage
capability)
- Verify the send button is disabled and placeholder reads "Attach an
image to edit..." when no image is attached
  - Attach an image and verify the form becomes submittable
- Select a text-to-image model and verify text-only prompts still
trigger generation normally
  - Ensure pressing `enter` doesn't bypass check

dashboard/src/lib/components/ChatForm.svelte

@@ -265,6 +265,7 @@
 
   function handleSubmit() {
     if ((!message.trim() && uploadedFiles.length === 0) || loading) return;
+    if (isEditOnlyWithoutImage) return;
 
     const content = message.trim();
     const files = [...uploadedFiles];
@@ -289,7 +290,11 @@
       if (imageFile.preview) {
         editImage(content, imageFile.preview);
       }
-    } else if (isImageModel() && content) {
+    } else if (
+      currentModel &&
+      modelSupportsTextToImage(currentModel) &&
+      content
+    ) {
       // Use image generation for text-to-image models
       generateImage(content);
     } else {