chore: remove Chromatic dependencies and configuration (#21221)

## Summary

- Remove `chromatic` and `@chromatic-com/storybook` devDependencies from
twenty-front
- Remove global `chromatic` Nx target from nx.json and twenty-front
project.json override
- Remove commented Chromatic Storybook addon from twenty-front
- Remove `CHROMATIC_PROJECT_TOKEN` from .env.example
- Update README to remove Chromatic sponsor reference (image was already
missing)
- Update stale Chromatic comment in toSpliced.ts

## Context

Visual regression testing has moved from Chromatic SaaS to self-hosted
Argos at `argos.twenty-internal.com`. These are dead references that are
no longer used by any CI workflow.

**Note:** Story `parameters.chromatic: { disableSnapshot: true }`
entries are intentionally kept — the Argos plugin reads them as a
fallback.

## Test plan

- Verify `yarn install` succeeds after dependency removal
- Verify no workflow references `chromatic` or `nx chromatic`

packages/twenty-front/src/utils/array/toSpliced.ts

@@ -6,7 +6,7 @@ type ToSplicedFn = {
 /**
  * Returns a new array with some elements removed and/or replaced at a given index.
  * This does the same as `Array.prototype.toSpliced`.
- * We cannot use the native `Array.prototype.toSpliced` method as of now because Chromatic's runners do not support it.
+ * Polyfill for environments that don't support Array.prototype.toSpliced.
  *
  * @param array - The array to remove and/or replace elements from.
  * @param start - The index at which to start changing the array.