chore: upgrade to svelte v5 with fix demo app

[Tony133]

Proposal:

• Upgrades the example app to svelte@^5.0.0, and fixes everything that broke.

Changes

• Svelte 5 dropped svelte/register (its CJS require-hook) and the compiler now only emits ESM, so .svelte files could no longer be require().

• Added example/svelte-loader.mjs, a Node module customization hook (module.register()) that compiles .svelte files to server-side JS on the fly, keeping their original file URLs so relative imports between components keep resolving normally without a bundler.

• example/worker.js also strips Svelte 5's hydration marker comments from SSR output (<!--[-->, <!--]-->), since Hotwire only ever swaps static HTML via Turbo and never hydrates client-side.

• Demo app fixes (unrelated to Svelte, surfaced by testing it):

  • /ws now registered inside a nested plugin, fixing a boot-order race with @fastify/websocket that left the websocket connection stuck pending forever.
  • superheroes@4's API change (randomSuperhero() instead of .random()).
  • POST /message now redirects instead of returning JSON, satisfying Turbo Drive's requirement for its target="_top" form.
  • GET / now reuses an existing session cookie instead of always minting a new identity, fixing an inability to delete a message you had just created.
  • POST /message/:id/delete now responds with a turbo-stream on success, consistent with its own error paths and /toast/ack.

Note:

• This PR should resolved this PR: chore: bump svelte from 3.59.2 to 5.56.4 #219 and this issue: Update to svelte v4 #105