dialog: Support configurable entrance animations

[xxww0098]

Description

Dialogs always slide from the window top to their resting position while fading in. When a dialog rests low in a tall window, it crosses much of the window before settling.

Add a shared DialogEntrance API so applications can choose the entrance for both Dialog and AlertDialog:

dialog.entrance(DialogEntrance::FadeSlide)
alert.entrance(DialogEntrance::Fade)

Entrance	Behavior
SlideDown (default)	Existing slide from the window top with its original timing and easing.
Fade	Fade in at the final position without movement.
FadeSlide	Fade in with short downward travel from above the resolved resting position.
None	Display the surface and backdrop immediately.

Fade and FadeSlide use the theme's normal duration and enter easing. FadeSlide uses the theme's short motion distance and applies travel after window-boundary positioning, so a dialog clamped upwards still moves relative to its actual resting position. Travel is shortened when there is insufficient space above the surface, and pointer hit testing follows the moving surface.

Reduced motion immediately displays the final state for every option. A zero theme duration also skips the fade entrances. Surface opacity, shadows, and the backdrop use the same entrance timing. Closing remains immediate.

The Dialog story exposes all four choices under Options → Entrance. Dialog and AlertDialog documentation is updated in English and Chinese.

Public API

gpui-component

• #[non_exhaustive] pub enum dialog::DialogEntrance { SlideDown, Fade, FadeSlide, None } — Select the shared entrance presentation. Implements Clone, Copy, Debug, Default, PartialEq, Eq, Serialize, and Deserialize.
• DialogEntrance::SlideDown — Default; preserve the existing slide and fade entrance.
• DialogEntrance::Fade — Fade at the final position without movement.
• DialogEntrance::FadeSlide — Fade with bounded, theme-defined downward travel.
• DialogEntrance::None — Immediately show the surface and backdrop.
• Dialog::entrance(self, entrance: DialogEntrance) -> Self — Choose the dialog entrance.
• AlertDialog::entrance(self, entrance: DialogEntrance) -> Self — Apply the same entrance policy to an alert dialog.

Breaking Changes

None for existing consumers. The API is additive and SlideDown preserves the existing default behavior.

Validation

• cargo test -p gpui-component --lib dialog -- --nocapture — 17 passed, covering resolved-boundary travel, intermediate positions, top-edge limits, pointer hit testing, builder defaults, AlertDialog forwarding, reduced motion, immediate keyboard dismissal, and zero theme duration.
• cargo check -p gpui-component-story — passed.
• cargo clippy -p gpui-component -p gpui-component-story --all-targets -- --deny warnings — passed.
• cargo fmt --all --check and git diff --check — passed.

Implementation, tests, and documentation were generated with AI assistance. The story was compile-checked; its animations were not manually compared across platforms.