feat(response-viewer): add a Copy Markdown button

The viewer renders the agent's answer, so selecting it loses the source:
headings, list markers and code fences do not survive. The only source copy
was the per-code-block button.

Copy Markdown copies the raw source of the newest answer the viewer holds:
every row of its turn, or the last response block when the reader emits no
turns. Tool and status blocks share the assistant role and are left out. The
copied text is recorded where the views render, so a reopened viewer never
copies an earlier answer, and an empty viewer warns instead of copying.

The feedback helper replaces the inline copy of the same logic in the
code-block button. On phones the header title now truncates so the longer
action row stays on screen.

.changeset/response-copy-markdown.md

@@ -0,0 +1,5 @@
+---
+"aicodeman": patch
+---
+
+Response viewer: add a Copy Markdown button that copies the raw source of the newest answer (every part of its turn), so headings, lists and code fences paste intact. An empty viewer and a refused clipboard both say so.