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/ToolIcon.tsx

@@ -0,0 +1,35 @@
+// The glyph column of a tool row's head. Recognition is narrow on purpose: an
+// unrecognised or absent `ToolKind` gets the generic bullet, never a blank
+// column and never a mark that stands for the wrong kind of work.
+
+import type { AcpToolKind } from "../../types.js";
+
+/** Glyph shown in a tool row's head, by `ToolKind`. */
+const KIND_GLYPH: Record<string, string> = {
+  read: "≡",
+  edit: "±",
+  delete: "✕",
+  move: "→",
+  search: "⌕",
+  execute: "$",
+  think: "…",
+  fetch: "⇣",
+  switch_mode: "⇄",
+};
+
+export function toolGlyph(kind: string | undefined): string {
+  return (kind && KIND_GLYPH[kind]) || "•";
+}
+
+export interface ToolIconProps {
+  kind: AcpToolKind | string | undefined;
+  className?: string;
+}
+
+export function ToolIcon({ kind, className = "st-tool-glyph" }: ToolIconProps) {
+  return (
+    <span className={className} aria-hidden="true">
+      {toolGlyph(kind)}
+    </span>
+  );
+}