…) + 1.6.0

Closes #627 — the cross-repo shuttle-widget chain is now complete.

## What

When the API (1.2.0+) emits ``widget: "shuttle_h"`` or ``"shuttle_v"``
on an M2M field (from ``ModelAdmin.filter_horizontal`` /
``filter_vertical``), the SPA now renders Django's two-pane
"available / chosen" shuttle widget instead of the single-list
checkbox bank. Scales well past the ~50-option ceiling where the
default falls over.

## Behaviour

- **Two panes** — Available (unselected) + Chosen (selected). Side-by-
  side for ``shuttle_h``, stacked for ``shuttle_v``. Mobile collapses
  both orientations to a single column.
- **Click to move** — clicking an item moves it to the other pane.
  Keyboard parity: Enter or Space on a focused option triggers the
  same move (the ``<li>`` carries ``role="option"`` + ``tabIndex=0``).
- **Per-pane search** — each pane has its own filter input that
  narrows the visible items client-side (no server roundtrip — the
  choices are already inlined in the wire payload).
- **"Choose all" / "Remove all"** — act on the FILTERED view, so
  typing a search term + clicking the link affects only those items.
- **Order preservation** — moving items INTO Chosen appends; moving
  items OUT keeps the relative order of survivors. Matches Django.
- **Numeric pk round-trip** — numeric choice values are emitted as
  numbers (not stringified) so the API receives the right type.

## Cross-repo chain

| Repo | Released |
|---|---|
| ``django-admin-rest-api`` 1.2.0 | [PyPI](https://pypi.org/project/django-admin-rest-api/1.2.0/) — emits the new hints |
| ``django-admin-react`` 1.6.0 | **this PR** — renders the widget |

SPA's ``django-admin-rest-api`` constraint tightens ``^1.1.0`` →
``^1.2.0`` so the new hint is guaranteed available; consumers on
1.1.x with this SPA build would see the silent-noop pattern
return (which is what landed #626 / #627 in the first place).

## Tests

Eight new vitests in ``ShuttleSelect.test.tsx``:
- Two-pane render (Available + Chosen).
- Click-to-move both directions.
- Per-pane search filter (case-insensitive).
- Choose all / Remove all bulk actions over the filtered view.
- Horizontal vs vertical orientation toggles the grid layout.
- Numeric pk round-trip.

## Verification

- ``poetry run pytest -q`` — **61 / 61 ✓** on Django 4.2.30
- ``pnpm test`` — **202 / 202 ✓** (up from 194; +8 new)
- ``pnpm -r typecheck`` ✓
- ``pnpm lint`` ✓
- ``pnpm -w build`` ✓

## Minor bump rationale

``1.5.1`` → ``1.6.0``. New user-visible widget per SemVer's
"additive features" guideline. Matches the symmetric ``1.2.0`` minor
bump on ``django-admin-rest-api``.

Closes #627.

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