Fix drawer backdrop exit transition on close

[xiao115255]

What & Why

Fixes #43024.

A closing Drawer dropped its native ::backdrop instantly. DialogBase closes the <dialog> synchronously, and leaving the top layer is what removes ::backdrop, so the backdrop never got a chance to transition. Dialog avoids this by overriding _shouldDeferClose() to keep the dialog in the top layer until the exit transition ends; Drawer did not override it, and was also missing the CSS that goes with it.

This makes Drawer mirror Dialog:

• js/src/drawer.ts — override _shouldDeferClose() (same as Dialog).
• scss/_drawer.scss — qualify the open-state selectors with :not(.hiding) so the exit falls back to the off-screen placement transform while [open] is still present, and add the .hiding::backdrop exit state so the backdrop fades out alongside the panel.

Two supporting fixes in js/src/dialog-base.ts, both reachable only because the close is now deferred:

• dispose() clears .hiding. _queueCallback() skips its completion callback once the instance is disposed (an SPA tearing the drawer down mid-navigation, or re-instantiating on the same element), which would strand the exit state. With [open]:not(.hiding) a stranded .hiding makes the next instance's drawer invisible while <html> stays scroll locked.
• The hide completion callback calls _closeAndCleanup() unconditionally. If something else closes the dialog during the exit (a <form method="dialog"> submit, app code calling dialog.close()), the previous if (element.open) guard skipped the cleanup and left dialog-open on <html> permanently. _closeAndCleanup() is already documented as safe on an already-closed dialog.

Note that Drawer now matches Dialog's exit behavior exactly, including that the dialog stays modal (and keeps swallowing pointer input) until the exit transition finishes.

How Tested

• npm run js-test-unit — 1355 tests pass across 33 files, coverage above the configured thresholds.
• npm run js-typecheck, npm run js-lint, npm run css-lint-stylelint, npm run css-test (47 specs), npm run dist — all pass.
• New unit tests: the drawer stays [open] and .hiding while the exit runs (fails without the _shouldDeferClose() override); instant drawers still close in the same task; dispose() mid-transition clears .hiding (fails without the base-class fix).
• End-to-end in headless Chromium against the compiled dist/ build, using the reproducer from the issue with --drawer-backdrop-bg overridden to make the backdrop visible, sampling getComputedStyle(dialog, '::backdrop') on the close click:
  • before: open and :modal both false 2 ms after the click, one distinct backdrop sample, no ::backdrop transition events;
  • after: [open] and :modal hold for the whole exit, background-color and backdrop-filter fire transitionrun/transitionend on ::backdrop, the alpha steps 0.25 → 0.14 → 0.07 → … → ~0 (10 distinct samples), and close() runs at ~298 ms.

AI Disclosure

This PR was prepared with AI assistance (Claude Code), reviewed and submitted by @xiao115255.