feat: support react-router fallbackElement and loading example

[ClarkXia]

This pull request introduces a new example demonstrating how to configure a custom fallbackElement for routing in an ice.js application, and adds support for specifying a fallbackElement in the router configuration. The changes include updates to the runtime to accept and use a custom fallback element, as well as the addition of a fully configured example app showcasing this feature.

[Copilot]

Pull Request Overview

This PR adds support for configuring a custom fallbackElement in ice.js router configuration. It allows developers to specify a React component that will be displayed while the router is loading new routes, providing a better user experience during navigation transitions.

• Extends the router configuration interface to accept a fallbackElement property
• Updates the runtime to use the configured fallback element in RouterProvider
• Adds a complete example demonstrating the feature with custom loading UI

Reviewed Changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
packages/runtime/src/types.ts	Adds fallbackElement property to router configuration interface
packages/runtime/src/appConfig.ts	Sets default fallbackElement value to null
packages/runtime/src/ClientRouter.tsx	Implements fallbackElement support in RouterProvider
examples/with-fallback-element/src/app.tsx	Demonstrates custom fallback element configuration
examples/with-fallback-element/src/pages/layout.tsx	Shows navigation state-based loading indicator
examples/with-fallback-element/src/pages/index.tsx	Basic test page with navigation link
examples/with-fallback-element/src/pages/home.tsx	Simple home page component
examples/with-fallback-element/package.json	Example project dependencies and scripts
examples/with-fallback-element/ice.config.mts	Example project configuration

Files not reviewed (1)

• pnpm-lock.yaml: Language not supported

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