Add Inertia.js API handler -> api.inertia

[bartekus]

Description:

This PR introduces first-class support for Inertia.js endpoints within Encore’s TypeScript runtime by adding a new api.inertia() helper method. This enables Encore applications to serve SSR-friendly single-page applications (SPAs) without leaving the Encore runtime model.

Key Changes:
Dependency Addition
runtimes/js/Cargo.toml

Adds the inertia-rust crate as a dependency to interface with Inertia’s protocol:

inertia = { git = "https://github.com/KaioFelps/inertia-rust", branch = "v2" }

New API Helper
runtimes/js/encore.dev/api/mod.ts

Adds api.inertia(), a new helper for defining Inertia-compatible endpoints:

export type InertiaHandler = (req, res) => Promise<{
  component: string;
  props: Record<string, any>;
  status?: number;
  version?: string;
}>;

api.inertia = function inertia(options, handler) { ... };

This API allows responding to both traditional page loads and X-Inertia JSON-based SPA transitions, offering a unified pattern for rendering pages server-side and client-side with minimal friction.

Why This Matters:

This addition aligns Encore with modern SSR/SPA architecture practices and makes it straightforward to integrate a frontend powered by Vue/React/Svelte via Inertia.js, without abandoning Encore’s opinionated backend design.

Example Use Case:

export const showDashboard = api.inertia(
  { path: "/dashboard", method: "GET", expose: true },
  async (req, res) => ({
    component: "Dashboard",
    props: { user: { name: "Alice" } },
  })
);

The endpoint seamlessly supports both direct HTML entry and client-side navigation with Inertia.

[encore-cla]

All committers have signed the CLA.