Feature: Global defaults for preserveScroll and preserveState

[weristwiegott]

Allow apps to configure global defaults so client-side navigations automatically use:
• preserveScroll: true
• preserveState: true

Motivation

Many apps (dashboards, CRUD heavy UIs, feed-like views) want a SPA feel by default:
• Keep component state between visits (forms, tabs, accordions).
• Don’t jump to top on every navigation.

Right now we have to repeat preserveScroll/preserveState:
• On every component
• Or every router.visit() call
This is noisy and error-prone, and teams inevitably forget it in some places.

Proposed API

1. Router-level defaults (framework-agnostic)

import { router } from '@inertiajs/core';

router.setDefaults({
  preserveScroll: true,
  preserveState: true,
});