Show minimal user feedback when a code-block Copy action fails #345

Problem

The message Copy button uses the shared clipboard helper and anchored feedback. The Markdown code-block Copy action calls navigator.clipboard.writeText directly; its failure callback in ChatMarkdown.tsx only logs to the console. A user can click Copy, have nothing copied, and receive no visible explanation.

Relevant code: apps/web/src/scient/presentation/CodeBlockActions.tsx, apps/web/src/components/ChatMarkdown.tsx, apps/web/src/hooks/useCopyToClipboard.ts, and apps/web/src/components/ui/anchoredCopyToast.ts.

This is an independent code-path gap, not the cause of the reported composer paste failure; that Copy action succeeded and the exact text reached the clipboard.

Desired behavior

Use Scient's existing reliable text-copy path and restrained anchored feedback where appropriate. On failure, show a small transient message near the Copy control, not a page-wide error. Keep the brief Copied success state and avoid duplicate notifications.

Validation

Test API success, unavailable/rejected API with fallback, complete failure, and unmount during an in-flight copy. Confirm the visible state matches actual clipboard success. Scope this to affected text-copy actions; do not rewrite unrelated binary or structured clipboard flows without evidence.