feat(canvas): add support for creating independent image nodes from expanded alternatives

docs/content/docs/progress/pending-test.mdx

@@ -15,6 +15,7 @@ The current release needs manual verification in these areas:
 - Multi-image primary selection: choosing another primary image should preserve the node center and current maximum edge while adapting to the new image ratio; free-resize nodes should keep their exact dimensions, and the viewport zoom indicator must not change.
 - Failed multi-image slots: every failed slot, including the current primary slot, should expose retry and delete actions. Retry should update only that slot without replacing an existing successful primary image; delete should remove only that failed slot and update the displayed count.
 - Multi-image resize: starting a resize while child images are expanded should collapse the group once, then resize the root node normally without triggering nested React updates or an application error.
+- Multi-image alternatives: every completed expanded alternative should provide clearly legible create-copy and set-primary actions in both themes; create-copy should add an independent image node to the right of the group, select it, and leave the original image group unchanged.
 - Canvas Agent startup, MCP status, model and permission controls, approvals, version reporting, diagnostics, streaming responses, history consistency, multi-tab isolation, and local Skill management.
 - Agent header tabs in English and Chinese at different panel widths; labels should collapse to icons and counts before either edge is clipped.
 - Agent message layout, Markdown, code blocks, attachments, token statistics, progress timelines, command groups, and scroll-follow behavior.