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`

nx.json

@@ -220,25 +220,6 @@
         }
       }
     },
-    "chromatic": {
-      "executor": "nx:run-commands",
-      "options": {
-        "cwd": "{projectRoot}",
-        "commands": [
-          {
-            "command": "nx storybook:build {projectName}",
-            "forwardAllArgs": false
-          },
-          "chromatic --storybook-build-dir=storybook-static {args.ci}"
-        ],
-        "parallel": false
-      },
-      "configurations": {
-        "ci": {
-          "ci": "--exit-zero-on-changes"
-        }
-      }
-    },
     "@nx/jest:jest": {
       "cache": true,
       "inputs": [