test: declare `canvas` so the jsdom tests get a real 2D context

captureDisplay.test.ts runs under @vitest-environment jsdom and says in its own
header that "jsdom + the `canvas` package give a real 2D context here" — but
canvas was never a direct devDependency. jsdom lists it as optional, so npm
skipped it, getContext('2d') returned null, and five tests died on
`ctx.fillStyle` with a TypeError. That aborted the pre-deploy test gate.

Declaring it directly makes the install deterministic; the lockfile already
carried the entry. All six tests pass with it present.

frontend/package.json

@@ -69,6 +69,7 @@
     "@vitejs/plugin-react": "^5.1.1",
     "@vitest/coverage-v8": "^4.0.18",
     "@vitest/ui": "^4.0.18",
+    "canvas": "^3.2.3",
     "eslint": "^9.39.1",
     "eslint-config-prettier": "10.1.8",
     "eslint-plugin-react-hooks": "^7.0.1",