feat(composer): carry reference image names and submit on Enter

Propagate an optional imageName through the agent content pipeline
(frontend types → API → domain → prompt building) so reference images
render with their name instead of a bare "image" label.

Also switch PromptComposer to submit on plain Enter (Shift+Enter for
newline, IME-safe) and add a submitDisabled guard wired from the canvas
and home composers.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/ui/components/CanvasAgentComposer/index.tsx

@@ -190,6 +190,7 @@ export const CanvasAgentComposer = forwardRef<PromptComposerHandle, CanvasAgentC
         onAnnotationPreviewChange={onAnnotationPreviewChange}
         onPasteFile={onUploadFile}
         onSubmit={onSubmit}
+        submitDisabled={generating || !canSubmit}
       />
       {previewSlot}
       <div className="assistant-composer-toolbar">