Plus lock-in tests for two audit verifications that already worked
(#629 / #636 — close-with-tests).

## #632 — toast colour by Django level

The detail page's `ObjectActionButton.onSuccess` was unconditionally
calling `toast.success(message || 'Done')` — so a
`message_user(..., level=messages.ERROR)` from a per-object action
toasted GREEN instead of red. The list page already dispatched by
level correctly; the asymmetry was hidden behind the legacy
`runObjectAction` adapter in `@dar/data/mutations.ts`, which collapsed
the `ActionRunResponse.messages[]` (each with a level) into a single
`message` string and dropped the level.

Fix:

- `ObjectActionRunResponse` now carries `messages?: UserMessage[]`
  alongside the legacy `message?: string`.
- `runObjectAction` propagates the full list.
- A new shared `toastMessages(toast, messages)` helper lives next to
  `useToast` and maps each level to the right toast method
  (`error` / `warning` → red, `info` / `debug` → blue,
  `success` / unknown → green) — mirrors what the list page already
  does inline.
- `DetailPage`'s `onSuccess` now uses the helper when `messages[]` is
  present; falls back to `toast.success(message || 'Done')` for
  back-compat with v1.4.x APIs that don't emit messages.

## #629 — prepopulated_fields verify-and-lock

The wire already carries `prepopulated_fields`; `CreatePage`
already slugifies the target from sources on each keystroke and
opts the target out of further auto-fill once the operator edits it
by hand. Pulled the logic out of `handleFieldChange` into a pure
`applyPrepopulate(...)` helper so a vitest can pin the four
behaviours (single source, joined sources, manual-edit lock, empty
fallback) without rendering the whole CreatePage.

## #636 — open-redirect protection on `?next=`

`DarLoginView` is a `LoginView` subclass that doesn't override
`get_success_url` / `form_valid` / `success_url_allowed_hosts`, so
the parent's same-host check on `?next=` is intact today. Added a
pytest that POSTs the login with `?next=https://evil.example.com/`
and asserts the redirect target is NOT off-host — locks the
property so a future override can't silently re-introduce the
open-redirect.

## Verification

- `pnpm -r typecheck` ✓
- `pnpm lint` (js + css + darkmode) ✓
- `pnpm test` — 174 / 174 (up from 163; +11 new) ✓
- `poetry run pytest tests/test_login_next_redirect.py -v` — 2 / 2 ✓
- `pnpm -w build` ✓

Closes #629, closes #632, closes #636.

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