feat(dashboard): add markdown preview for AskUserQuestion options

Implement side-by-side preview layout when AskUserQuestion options
include markdown content, matching the Claude Code tool spec.

Hook changes (bin/amc-hook):
- Extract optional 'markdown' field from question options
- Only include when present (maintains backward compatibility)

QuestionBlock layout:
- Detect hasMarkdownPreviews via options.some(opt => opt.markdown)
- Standard layout: vertical option list (unchanged)
- Preview layout: 40% options left, 60% preview pane right
- Fixed 400px preview height prevents layout thrashing on hover
- Track previewIndex state, update on mouseEnter/focus

Content rendering (smart detection):
- Code fences (starts with ```): renderContent() for syntax highlighting
- Everything else: raw <pre> to preserve ASCII diagrams exactly
- "No preview for this option" when hovered option lacks markdown

OptionButton enhancements:
- Accept selected, onMouseEnter, onFocus props
- Visual selected state: brighter border/bg with subtle shadow
- Compact padding (py-2 vs py-3.5) for preview layout density
- Graceful undefined handling for standard layout usage

Bug fixes during development:
- Layout thrashing: Fixed height (not max-height) on preview pane
- ASCII corruption: Detect code fences vs plain text for render path
- Horizontal overflow: Use overflow-auto (not just overflow-y-auto)
- Data pipeline: Hook was stripping markdown field (root cause)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

bin/amc-hook

@@ -82,10 +82,14 @@ def _extract_questions(hook):
             "options": [],
         }
         for opt in q.get("options", []):
-            entry["options"].append({
+            opt_entry = {
                 "label": opt.get("label", ""),
                 "description": opt.get("description", ""),
-            })
+            }
+            # Include markdown preview if present
+            if opt.get("markdown"):
+                opt_entry["markdown"] = opt.get("markdown")
+            entry["options"].append(opt_entry)
         result.append(entry)
     return result