feat(Modal): add nesting behaviors and fix third-party overlay dismissal

.changeset/modal-nested-third-party-overlays.md

@@ -0,0 +1,5 @@
+---
+"@easypost/easy-ui": patch
+---
+
+Fix third-party overlays (e.g. Stripe Link/autofill) locking up when their modal (`allowsThirdPartyOverlays`) is nested inside another modal. A surrounding focus-trapping modal kept hiding the page (`inert`) and containing focus, which re-broke the injected overlay. A modal now automatically relaxes its focus trap and background hiding while it has an open `allowsThirdPartyOverlays` descendant, then restores them when that descendant closes. Modals without such a descendant are unaffected.

.changeset/modal-third-party-overlay-dismiss.md

@@ -0,0 +1,5 @@
+---
+"@easypost/easy-ui": patch
+---
+
+Fix react-aria overlays (e.g. `Select`, `Menu`) not closing on outside click when rendered inside a `Modal` that uses `allowsThirdPartyOverlays`. The modal box was tagged `data-react-aria-top-layer`, which made react-aria treat every click inside the modal as "not outside" for all overlays. Modals now keep nested overlays dismissable while preserving the third-party overlay behavior: the modal stays visible under a surrounding modal, clicking a nested modal no longer dismisses the one beneath it, and genuine third-party overlays (e.g. Stripe) still don't dismiss the modal.

.changeset/nested-modal-behavior.md

@@ -0,0 +1,7 @@
+---
+"@easypost/easy-ui": minor
+---
+
+Add `childNestingBehavior` and `selfNestingBehavior` props to `Modal.Trigger` and `ModalContainer` to control how modals stack when nested. The connection between a parent and a nested child can resolve to `stack` (both keep their backdrops — the default) or `replace` (the nested modal hides the modal beneath it).
+
+Configure it from either end: `childNestingBehavior` is set on the parent, applies to its children, and cascades down the tree; `selfNestingBehavior` is set on the child, applies only to its own connection to its parent, does not cascade, and overrides the parent's `childNestingBehavior` for that connection. `selfNestingBehavior` is useful for surgically changing one nested modal in a large tree without touching its parent.

easy-ui-react/package.json

@@ -29,6 +29,7 @@
     "test:watch": "vitest"
   },
   "dependencies": {
+    "@react-aria/overlays": "^3.31.0",
     "@easypost/easy-ui-icons": "1.0.0-alpha.55",
     "@easypost/easy-ui-tokens": "1.0.0-alpha.17",
     "@react-aria/toast": "^3.0.9",

easy-ui-react/src/Modal/Modal.mdx

@@ -133,6 +133,77 @@ Use this sparingly. It trades away the modal's focus containment and background
 </Modal.Trigger>
 ```
 
+### Nesting a third-party overlay under another modal
+
+A modal with `allowsThirdPartyOverlays` doesn't run focus trapping or background hiding itself, but a **surrounding** focus-trapping modal would — and react-aria keeps the surrounding modal's page-hiding active even while the nested one is open, which would re-`inert` the third-party overlay (e.g. Stripe Link) and steal focus back.
+
+This is handled automatically: a focus-trapping modal relaxes its focus trap and background hiding for as long as it has an open `allowsThirdPartyOverlays` descendant (at any depth), then restores them when that descendant closes. You only need to set `allowsThirdPartyOverlays` on the modal that hosts the overlay — no configuration is required on its ancestors.
+
+```tsx
+{/* The outer modal relaxes automatically while the nested Stripe modal is open. */}
+<Modal.Trigger>
+  <Button>Open</Button>
+  <Modal>
+    <Modal.Header>Billing</Modal.Header>
+    <Modal.Body>
+      <Modal.Trigger allowsThirdPartyOverlays>
+        <Button>Add card</Button>
+        <Modal>
+          <Modal.Header>New Credit Card</Modal.Header>
+          <Modal.Body>{/* Stripe CardElement, etc. */}</Modal.Body>
+        </Modal>
+      </Modal.Trigger>
+    </Modal.Body>
+  </Modal>
+</Modal.Trigger>
+```
+
+## Nesting behavior
+
+When modals nest, the connection between a parent modal and its nested child can present in one of two ways:
+
+- `stack` (the default) — both modals keep their own backdrops; modals simply stack.
+- `replace` — the nested modal hides the modal beneath it, so only the topmost modal is visible.
+
+You configure that connection from either end, on `<Modal.Trigger />` or `<ModalContainer />`:
+
+- **`childNestingBehavior`** is set on the parent. It applies to its nested children and **cascades** down the tree, so setting it once at the top governs the whole stack. A descendant that doesn't set its own inherits it.
+- **`selfNestingBehavior`** is set on the child. It applies only to that modal's connection to its parent, is **local** (does not cascade), and **overrides** the parent's `childNestingBehavior` for that one connection.
+
+```tsx
+{/* Set once at the top; every nested modal replaces the one beneath it. */}
+<Modal.Trigger childNestingBehavior="replace">
+  <Button>Open modal</Button>
+  <Modal>
+    <Modal.Header>Title</Modal.Header>
+    <Modal.Body>{/* each nested modal takes over the one beneath it */}</Modal.Body>
+  </Modal>
+</Modal.Trigger>
+```
+
+### Configuring a single nested modal
+
+When you can't (or don't want to) change the parent — for example one nested modal deep in a larger tree of modals that should take over its parent — set `selfNestingBehavior` on the nested modal instead. It changes only that modal's connection to its parent, without touching the parent's configuration.
+
+```tsx
+<Modal.Trigger>
+  <Button>Open parent</Button>
+  <Modal>
+    <Modal.Header>Parent</Modal.Header>
+    <Modal.Body>
+      {/* This nested modal takes over the parent without touching the parent. */}
+      <Modal.Trigger selfNestingBehavior="replace">
+        <Button>Open nested</Button>
+        <Modal>
+          <Modal.Header>Nested</Modal.Header>
+          <Modal.Body>{/* … */}</Modal.Body>
+        </Modal>
+      </Modal.Trigger>
+    </Modal.Body>
+  </Modal>
+</Modal.Trigger>
+```
+
 ## Properties
 
 ### Modal.Trigger