feat(maestro): port the structured pane to React

Closes #246. The transcript, composer and approval controls move from
structured.ts/structuredRender.ts into components under
web/src/components/structured/, against docs/STRUCTURED-VIEW.md: eight row
kinds, both projections, the single-active approval nonce, and diff rendering
with its own line counts.

The pure layers stay shared — structuredDiff.ts computes the diff,
structuredRender.ts still sanitizes markdown — and model.ts holds the
client-side decisions (activity grouping, notice classification, offered
decisions, `/` completion) so they are testable without a DOM.

Like TerminalPane, the pane draws the body only; pane chrome reads the
connection through onStatusChange.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

web/src/components/structured/ToolGroup.tsx

@@ -0,0 +1,43 @@
+// A turn's collapsed activity block: two or more tool rows after the same
+// `User` row, plus the narration absorbed between them. The header counts tool
+// rows only (docs/STRUCTURED-VIEW.md); opening it reveals tools and text in
+// chronological order.
+
+import type { ReactNode } from "react";
+
+export interface ToolGroupProps {
+  id: string;
+  toolCount: number;
+  expanded: boolean;
+  onExpandedChange(expanded: boolean): void;
+  children: ReactNode;
+}
+
+export function ToolGroup({ id, toolCount, expanded, onExpandedChange, children }: ToolGroupProps) {
+  return (
+    <div className="st-tool-group" data-tool-group-id={id}>
+      <button
+        type="button"
+        className="st-tool-group-head"
+        aria-expanded={expanded}
+        onClick={() => onExpandedChange(!expanded)}
+      >
+        <span className="st-tool-group-glyph" aria-hidden="true">
+          ›
+        </span>
+        <span className="st-tool-group-title">{`${toolCount} tool calls`}</span>
+      </button>
+      <div
+        className={
+          expanded ? "st-tool-group-body st-disclosure is-expanded" : "st-tool-group-body st-disclosure"
+        }
+        aria-hidden={!expanded}
+        inert={!expanded}
+      >
+        <div className="st-disclosure-clip">
+          <div className="st-disclosure-inner">{children}</div>
+        </div>
+      </div>
+    </div>
+  );
+}