Closes #622 — the cross-repo Django 4.2 chain is now complete.

## What

Relax this package's Django pin from `>=5.0,<7.0` to `>=4.2,<7.0`,
bump the API + MCP dep constraints to their 4.2-aware floors,
add a 4.2 dimension to CI, and update the README's Requirements
section + classifier list to advertise 4.2.

| Repo | Before | After | Released |
|---|---|---|---|
| `django-admin-rest-api` | `django >=5.0` | `django >=4.2` | 1.1.0 (PyPI) |
| `django-admin-mcp-api`  | `django >=5.0` | `django >=4.2` | 1.1.0 (PyPI) |
| `django-admin-react`    | `django >=5.0` | `django >=4.2` | **1.5.0 (this PR)** |

Why the constraint tightens on `django-admin-mcp-api` even though
its public range stays the same: the 1.0.x line pins
``django>=5.0``, so a consumer on Django 4.2 would resolve to an
mcp version that refuses to install. Bumping the floor to
``>=1.1.0`` keeps the install promise honest.

## CI matrix

Added a `matrix.django: ["4.2", "5.2"]` dimension; runtime install
swaps Django via the venv's `pip install "django~=<matrix>.0"`.
Today both matrix cells run py3.12.

## README freshness

While here, two doc updates aligned to the post-#631 reality:

- `PRIMARY_COLOR` default is now `None` (the resolver reads
  `AdminSite.site_primary_color` next, falling back to the
  built-in `#2563eb`). The settings-block example reflects the
  None default.
- New "Accent colour" subsection in Branding documents the full
  resolution order + the `site_primary_color` AdminSite-attr
  convention, with a worked `AcmeAdminSite` example.

The "Requirements" line now reads `Django: 4.2 LTS, 5.0, 5.1,
5.2 LTS, 6.0 (and any later 6.x)` — accurate for what the wheel
will actually install on.

## Verification

- `poetry run pytest -q` — **61 / 61 ✓** on Django 4.2.30
- `poetry run pytest -q` — **61 / 61 ✓** on Django 5.2.14 (no regression)
- `pnpm test` — **187 / 187 ✓**
- `pnpm -r typecheck` ✓
- `pnpm lint` ✓
- Cross-repo install resolved end-to-end: rest-api 1.1.0 + mcp-api
  1.1.0 + django 4.2.30 + this package 1.5.0 in one venv.

## Minor bump rationale

`1.4.13` → `1.5.0`. New supported environment (Django 4.2 LTS)
per SemVer's "additive features that broaden compatibility"
guideline. Matches the symmetric 1.1.0 minor bumps on
`django-admin-rest-api` and `django-admin-mcp-api`.

Closes #622.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>