feat(overlays): register open dialogs in an overlay stack

[kelsos]

Closes #579.

A RuiDialog is not a history entry, so a back gesture pops the entry underneath it and leaves the page it was sitting on; with dialogs stacked, one press skips all of them. A consumer can keep its own registry, but every dialog has to opt in by hand and each new one silently regresses.

The API

const { hasOverlay, dismissTop } = useOverlayStack();
useDismissableOverlay(open, dismiss);   // for anything the library doesn't already register

RuiDialog registers itself while open, and RuiBottomSheet comes along because it renders one.

What the contract has to get right

All three of these come from the app-side version and are covered by tests:

• Dismissing is a request the layer may decline, so nothing is removed from the stack on dismissal. Layers leave when their own open state says so, and dismissTop looks for the topmost one still showing. Removing on dismissal deregisters a dialog still covering the page, and the next gesture sails past it.
• Registration follows modelValue alone, never persistent. A dialog that turns persistent partway through its life has to stay in the stack and refuse, or the gesture starts passing through exactly once there is work to lose.
• dismissTop reports whether it handled the gesture, and a layer that refuses still counts as handled.

Where this departs from the issue, and why

Issue	Here	Reason
hasOverlay = anything registered	anything still showing	Nothing is removed on dismissal, so the registered count reports a dialog as up after it has gone. This is also why a layer stores a getter rather than a boolean: a computed over a plain closure would never re-evaluate.
closeTop	dismissTop	A persistent dialog answers without closing, so "close" is the wrong promise for a public API.
register in RuiDialog and RuiBottomSheet	RuiDialog only	RuiBottomSheet renders a RuiDialog, so doing both puts every sheet in the stack twice.
order from zIndex	order from registration	RuiDialog.vue:38 defaults zIndex to 9999 for every dialog, so it says nothing about which is on top.

Two additions the issue didn't call for: a dismiss emit, so the gesture is observable without reusing click:esc and lying about where it came from; and resetOverlayStack(), which the unit project needs because its pool shares one module registry across the files in a worker.

Telling a pop from a push stays out of the library

As the issue argues, the router wiring is the consumer's business. The example app shows it, and the interesting part is what does not work: a raw popstate listener double-fires, because vue-router's restoration of an aborted pop is itself a pop and reads as a second gesture — which closed two dialogs per press when I first wired it that way. Taking the direction from vue-router's own history listener is what works:

router.options.history.listen((_to, _from, info) => { direction = info.direction; });
router.beforeEach(() => {
  const isBack = direction === 'back';
  direction = undefined;
  return !isBack || !dismissTop();
});

Verified

Lint (0 errors), both typecheck projects, 1429 unit tests (16 new across two specs), 468 storybook tests, 388 e2e tests, build:prod.

The e2e drives real goBack() gestures on a new /overlay-stack page: nested dialogs come down one at a time before the page is left, a persistent dialog swallows two gestures and refuses both, and a link inside a dialog still navigates.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.41%. Comparing base (d44e8ea) to head (8fb7d45).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #584      +/-   ##
==========================================
+ Coverage   87.33%   87.41%   +0.07%     
==========================================
  Files         168      169       +1     
  Lines        6522     6563      +41     
  Branches     2057     2062       +5     
==========================================
+ Hits         5696     5737      +41     
  Misses        826      826              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.