refactor(canvas): install wheel zoom via a passive-safe native listener

Move canvas wheel zoom off the React onWheel prop into a manually attached
non-passive listener so preventDefault reliably suppresses page scroll, and
read the viewport from the ref to keep the handler referentially stable.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/tests/canvasWheel.test.mjs

@@ -0,0 +1,29 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import { installCanvasWheelListener } from "../src/ui/pages/CanvasWorkspace/canvas/canvasWheel.ts";
+
+test("installs the canvas wheel handler as non-passive and removes it cleanly", () => {
+  const calls = [];
+  const target = {
+    addEventListener(type, listener, options) {
+      calls.push({ action: "add", type, listener, options });
+    },
+    removeEventListener(type, listener) {
+      calls.push({ action: "remove", type, listener });
+    }
+  };
+  const listener = () => {};
+
+  const cleanup = installCanvasWheelListener(target, listener);
+
+  assert.equal(calls[0].action, "add");
+  assert.equal(calls[0].type, "wheel");
+  assert.strictEqual(calls[0].listener, listener);
+  assert.deepEqual(calls[0].options, { passive: false });
+
+  cleanup();
+  assert.equal(calls[1].action, "remove");
+  assert.equal(calls[1].type, "wheel");
+  assert.strictEqual(calls[1].listener, listener);
+});