feat(dialog): clearer open and close motion

[itsprade]

Summary

Draft for discussion: makes Dialog open and close read more clearly. The change touches only the shared dialog, so every modal moves the same way.

• Open: keeps the existing 200ms fade and zoom-in, and adds a small upward slide (8px), eased out.
• Close: quicker, at 150ms eased in, with a matching slide down. The overlay closes on the same timing.
• Reduced motion: both are skipped under prefers-reduced-motion.

CSS only, with existing tw-animate-css utilities and no new dependency. This is in line with the CSS-first animation direction (platform-planning#1510).

Split out of the TreePicker / LineItems work (#597, #595). Component-specific motion, like the TreePicker's accordion, stays in those PRs; the shared dialog-level change lives here.

Open questions

• Are slide distance and timings right, or should this wait for the motion tokens (--duration-*, --ease-*) planned in #1510?
• Should Sheet and Popover get matching treatment in the same pass?

Test plan

• Dialog snapshot updated; full suite passes
• Checked on /showcase/primitives: open is 200ms ease-out with the slide, close is 150ms ease-in

🤖 Generated with Claude Code

[github-actions]

Code Metrics Report

	main (403eebe)	#598 (3d0d032)	+/-
Coverage	88.1%	88.1%	0.0%
Test Execution Time	2m18s	2m19s	+1s

Details

  |                     | main (403eebe) | #598 (3d0d032) | +/-  |
  |---------------------|----------------|----------------|------|
  | Coverage            |          88.1% |          88.1% | 0.0% |
  |   Files             |            207 |            207 |    0 |
  |   Lines             |           6178 |           6178 |    0 |
  |   Covered           |           5448 |           5448 |    0 |
- | Test Execution Time |          2m18s |          2m19s |  +1s |

Code coverage of files in pull request scope (95.2% → 95.2%)

Files	Coverage	+/-	Status
packages/core/src/components/dialog/dialog.tsx	95.2%	0.0%	modified

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Reported by octocov