…picker

Code blocks in chat previously had a single hover-revealed Copy button.
Replace it with a richer toolbar:

1. Copy as Markdown: wraps the value in a fenced code block (```lang) so
   the snippet pastes into a GitHub issue or README with full syntax
   highlighting preserved. The leading fence uses the current effective
   language (override, then prop, then the workspace's remembered-most-
   recent pick). Brief check-mark feedback reuses the existing pattern.

2. Language picker: a small chevron button next to the copy buttons.
   Opens a 30-entry menu of the most common languages the agent emits.
   The user's manual pick is persisted per-workspace (last 5 picks) in
   localStorage and used as the smart default for the next unlabelled
   block in that workspace. The override is held in CodeViewer state so
   highlighter re-runs the right hljs grammar; an Escape or outside click
   closes the picker without committing.

3. Reserved slots: the toolbar is shaped to accept 'Run in workspace' and
   'Open in editor' actions as future bridge additions — the layout
   doesn't need to change to add them.

The toolbar wrapper is a flex row at top-right of the code block; the
existing copy button keeps its position. The whole cluster is opacity-0
by default and reveals on :hover/:focus-within with an 80ms delay on the
fade-out so a mouse pass over the picker list doesn't blink the buttons
away mid-hover. prefers-reduced-motion is honored through the existing
@media rule.

New files: lib/codeBlockActions.ts (LANGS + rememberLang + suggestedLang
+ languageLabel), components/CodeBlockToolbar.tsx.