feat(useStack): add app-wide defaultTeleport option to createStackPlugin

[sridhar-3009]

Closes #553

Problem

Today the only way to teleport all Portals to a non-default target is per-component: every <SnackbarPortal to="top-layer" />, every Dialog, every Toast must opt in individually. There is no single place to configure the fallback for the whole app.

Solution

Add a default option to createStackPlugin (and to createStack/StackOptions):

app.use(createStackPlugin({ default: 'top-layer' }))

Portal reads stack.default as its fallback when no to prop is given, resolving the precedence chain:

per-component `to` prop  →  stack.default  →  'body'

'top-layer' is handled identically to the per-component case — it resolves to stack.topElement.value (the topmost open <dialog>) and falls back to 'body' when no modal is active.

Changes

• StackOptions: new default?: Extensible<'top-layer'> | HTMLElement field
• StackContext: new default: Readonly<Ref<…>> property
• createStack(): extracts and wraps default in a toRef, exposes it on the returned context
• Portal.vue: to prop destructured without a hardcoded 'body' default; target resolution now: to ?? stack.default.value ?? 'body'
• Tests: createStack({ default }) exposes the value; createStackPlugin({ default }) flows through to useStack().default

[github-actions]

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