fix(react-ui): reflect disabled state on SearchableSelect button

The Backend dropdown is disabled while /backends/known is in flight
(disabled={isSubmitting || backendsLoading} in ImportModel.jsx). Until
now the disabled prop only guarded the internal onClick handler — there
was no `disabled` HTML attribute on the <button>, so the element
remained "actionable" from the outside.

That regressed the import-form-ux Batch D / E Playwright tests after
the i18next-suspense PR (#9642): suspending on the importModel
namespace defers the useEffect that fetches /backends/known, so when
the test calls backendTrigger.click() the button is rendered but
backendsLoading is still true. The click hits the no-op branch,
the dropdown stays closed, and `getByRole('option', { name: 'llama-cpp' })`
times out at 30s.

Surfacing the disabled state on the actual <button> makes Playwright
auto-wait until the dropdown is ready, fixes a11y (screen readers now
announce "disabled"), and removes the button from the tab order while
loading.

Assisted-by: Claude:claude-opus-4-7[1m] [Claude Code]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>