chore: replace isChromatic with isPixel (#26832)

Swaps `chromatic/isChromatic` for `@coder/pixel-storybook`'s `isPixel()`
so the
snapshot-determinism gates (fixed workspace name, frozen Spinner, fixed
CLI
origin, no scroll, font loader) fire under pixel instead of Chromatic.
Drops the
now-unused `chromatic` dependency.

Stacked on #27658 (pixel-storybook 0.3); `isPixel` comes from the new
`@coder/pixel-storybook/storyapi` subpath. Verified the app and
Storybook
builds both bundle `isPixel` cleanly, and `tsc` passes with the
dependency
removed.

This is the last piece of the Chromatic removal; the story params
migrated in
#26844 and the addon came out in #27353.

<details>
<summary>Chromatic removal sequence</summary>

1. Remove the Chromatic CI job + scripts + docs reference. (#26777,
merged)
2. **This PR** — `isChromatic()` → `isPixel()`; drop the `chromatic`
dependency.
3. `data-pixel` + `pixel.exclude`; drop `delay` / `pauseAnimationAtEnd`.
(#26778,
   merged)
4. Migrate story snapshot params (`viewports` / `diffThreshold` / theme
modes →
   `pixel.matrix`); delete `testHelpers/chromatic.ts`. (#26844, merged)
5. Remove the `@chromatic-com/storybook` addon. (done on main in #27353)

</details>

---

> Generated by Coder Agents on behalf of @aslilac.

site/package.json

@@ -161,7 +161,6 @@
 		"@vitest/browser-playwright": "4.1.7",
 		"autoprefixer": "10.5.0",
 		"babel-plugin-react-compiler": "1.0.0",
-		"chromatic": "11.29.0",
 		"dpdm": "3.15.1",
 		"express": "4.21.2",
 		"jest-canvas-mock": "2.5.2",