feat(canvas): keyboard shortcuts + toolbar undo/redo buttons

UI-facing half of the undo/redo feature. Combined with the previous two
commits, Ctrl+Z (or the toolbar button) now reverses every canvas
mutation: add/remove component, move, rotate, set property, add/remove
wire.

EditorPage.tsx:
- New window-keydown effect for Ctrl+Z / Ctrl+Y / Ctrl+Shift+Z (and the
  Cmd equivalents). Uses the same input/textarea/contenteditable guard
  as the existing Ctrl+S handler — Monaco's per-file undo and the AI
  chat composer keep their own behaviour.

SimulatorCanvas.tsx:
- Two icon buttons (undo + redo) added to the canvas header, between
  the board selector and the Serial Monitor toggle. Tooltip surfaces
  the next command's description ("Undo: Add LED (Ctrl+Z)") so the
  user knows exactly what's about to revert. Buttons disable when the
  stack is empty in that direction.
- New `canvas-icon-btn` CSS class for square 32×32 icon-only buttons
  (matches the visual weight of the existing Serial button without
  the label).
- Subscribes to history / historyIndex via store selectors so the
  buttons re-render reactively as commands are pushed/undone.

No new tests — the store-level coverage from 99ed22b already exercises
undo/redo round trips. UI affordances are wired pass-through to those
store APIs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/components/simulator/SimulatorCanvas.css

@@ -137,6 +137,36 @@
   color: #4fc3f7;
 }
 
+/* ── Canvas icon-only buttons (undo/redo, …) ─────── */
+.canvas-icon-btn {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 32px;
+  height: 32px;
+  background: transparent;
+  border: 1px solid #3a4a5a;
+  border-radius: 5px;
+  color: #9d9d9d;
+  cursor: pointer;
+  transition:
+    border-color 0.15s,
+    background 0.15s,
+    color 0.15s,
+    opacity 0.15s;
+  flex-shrink: 0;
+}
+
+.canvas-icon-btn:hover:not(:disabled) {
+  border-color: #007acc;
+  color: #ccc;
+}
+
+.canvas-icon-btn:disabled {
+  opacity: 0.4;
+  cursor: not-allowed;
+}
+
 /* ── Component count ─────────────────────────────── */
 .component-count {
   display: flex;

frontend/src/components/simulator/SimulatorCanvas.tsx

@@ -143,6 +143,12 @@ export const SimulatorCanvas = ({ headerSlot }: SimulatorCanvasProps = {}) => {
   const recordRotate = useSimulatorStore((s) => s.recordRotate);
   const recordSetProperty = useSimulatorStore((s) => s.recordSetProperty);
   const recordRemoveWire = useSimulatorStore((s) => s.recordRemoveWire);
+  // Subscribe to history shape so the undo/redo buttons reactively
+  // enable/disable and their tooltips reflect the next command.
+  const history = useSimulatorStore((s) => s.history);
+  const historyIndex = useSimulatorStore((s) => s.historyIndex);
+  const undo = useSimulatorStore((s) => s.undo);
+  const redo = useSimulatorStore((s) => s.redo);
 
   // Oscilloscope
   const oscilloscopeOpen = useOscilloscopeStore((s) => s.open);
@@ -1901,6 +1907,61 @@ export const SimulatorCanvas = ({ headerSlot }: SimulatorCanvasProps = {}) => {
               </span>
             )}
 
+            {/* Undo / Redo — canvas-scoped, mirrors the Ctrl+Z / Ctrl+Y
+                handler in EditorPage. Tooltip surfaces the description of
+                the command that would be applied. Disabled when the stack
+                is exhausted in that direction. */}
+            <button
+              onClick={() => undo()}
+              disabled={historyIndex < 0}
+              className="canvas-icon-btn"
+              title={
+                historyIndex >= 0
+                  ? `Undo: ${history[historyIndex].description} (Ctrl+Z)`
+                  : 'Nothing to undo'
+              }
+              aria-label="Undo"
+            >
+              <svg
+                width="18"
+                height="18"
+                viewBox="0 0 24 24"
+                fill="none"
+                stroke="currentColor"
+                strokeWidth="2"
+                strokeLinecap="round"
+                strokeLinejoin="round"
+              >
+                <path d="M3 7v6h6" />
+                <path d="M3 13a9 9 0 1 0 3-7.7L3 8" />
+              </svg>
+            </button>
+            <button
+              onClick={() => redo()}
+              disabled={historyIndex >= history.length - 1}
+              className="canvas-icon-btn"
+              title={
+                historyIndex < history.length - 1
+                  ? `Redo: ${history[historyIndex + 1].description} (Ctrl+Y)`
+                  : 'Nothing to redo'
+              }
+              aria-label="Redo"
+            >
+              <svg
+                width="18"
+                height="18"
+                viewBox="0 0 24 24"
+                fill="none"
+                stroke="currentColor"
+                strokeWidth="2"
+                strokeLinecap="round"
+                strokeLinejoin="round"
+              >
+                <path d="M21 7v6h-6" />
+                <path d="M21 13a9 9 0 1 1-3-7.7L21 8" />
+              </svg>
+            </button>
+
             {/* Serial Monitor toggle */}
             <button
               onClick={() => {

frontend/src/pages/EditorPage.tsx

@@ -197,6 +197,34 @@ export const EditorPage: React.FC = () => {
     return () => window.removeEventListener('keydown', handler);
   }, [handleSaveClick]);
 
+  // Ctrl+Z / Ctrl+Y / Ctrl+Shift+Z — canvas undo/redo. Skipped when the
+  // user is typing in any input/textarea/contenteditable so the Monaco
+  // editor's per-file history (and the AI chat composer, etc.) keep
+  // working untouched.
+  useEffect(() => {
+    const handler = (e: KeyboardEvent) => {
+      const t = e.target as HTMLElement | null;
+      if (t) {
+        const tag = t.tagName;
+        if (tag === 'INPUT' || tag === 'TEXTAREA' || tag === 'SELECT' || t.isContentEditable) {
+          return;
+        }
+      }
+      if (!(e.ctrlKey || e.metaKey)) return;
+      const k = e.key.toLowerCase();
+      const sim = useSimulatorStore.getState();
+      if (k === 'z' && !e.shiftKey) {
+        e.preventDefault();
+        sim.undo();
+      } else if (k === 'y' || (k === 'z' && e.shiftKey)) {
+        e.preventDefault();
+        sim.redo();
+      }
+    };
+    window.addEventListener('keydown', handler);
+    return () => window.removeEventListener('keydown', handler);
+  }, []);
+
   // Prevent body scroll on the editor page
   useEffect(() => {
     const html = document.documentElement;