feat(clipboard): paste images and text from the system clipboard

Replace the internal single-node clipboard with real clipboard paste:
the prompt composer uploads pasted image files as references, and the
canvas pastes clipboard images as nodes and clipboard text as text
nodes anchored at the pointer, reading from the async clipboard API on
the context-menu paste action.

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

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

@@ -188,6 +188,7 @@ export const CanvasAgentComposer = forwardRef<PromptComposerHandle, CanvasAgentC
         onReferenceRemoved={onReferenceRemoved}
         onAnnotationsChange={onAnnotationsChange}
         onAnnotationPreviewChange={onAnnotationPreviewChange}
+        onPasteFile={onUploadFile}
         onSubmit={onSubmit}
       />
       {previewSlot}