Remove dead done prop from carousel slides

[evnchn]

Motivation

Carousel._handle_value_change sets a :done prop on every slide on each value change, but Quasar's QCarouselSlide has no done prop — it's a dead no-op (copy-pasted from Stepper, where done is a real QStep prop).

Minimal reproduction (nicegui/elements/carousel.py:43):

from nicegui import ui
c = ui.carousel(value='a')
with c:
    ui.carousel_slide('a'); ui.carousel_slide('b')
c.value = 'b'                                   # fires _handle_value_change
print(any(':done' in s._props for s in c))      # True on origin/main -> dead attr on slides

→ True (a stray :done attribute Quasar ignores)

Implementation

Remove the dead _handle_value_change override. The base ValueElement._handle_value_change still syncs model-value, so selection behaviour is unchanged.

Dead-code provenance (git blame) & third-party safety

• Origin: introduced with ui.carousel — Falko Schindler, decdf97 (2023-07-16). The done loop is copy-pasted from Stepper (where done is a real QStep prop); QCarouselSlide has never had one. So this is author-copy-paste-error, not a feature obsoleted by later changes.
• Third-party side-effect check (does the :done write trigger a re-render that surfaces recently-added user content?): slide.props(':done=…') does call slide.update() when the done-state flips — but update() flushes only that slide element's own props (the inert :done attribute); it does not re-render children. Recently-added user content is surfaced independently: NiceGUI auto-enqueues the parent's update whenever a child is created (element.py:85,87 → enqueue_update(parent_slot.parent)). So removing this override cannot hide user content — confirmed by tracing the update path.

Verification

• No pytest. The prop is inert, so removing it changes nothing observable — there is no user-visible behaviour for a test to guard. An earlier revision asserted on slide._props; per Prefer behavior tests over tests coupled to implementation details #6219 that is a test coupled to internals rather than to behaviour, so it was dropped.
• Local gates: pre-commit · mypy ./nicegui · pylint 10.00/10 · pytest — all green.

Progress

• The PR title is a short phrase starting with a verb.
• The implementation is complete.
• This PR does not address a security issue.
• Pytests are not necessary.
• Documentation is not necessary.
• No breaking changes to the public API.