chore: add webui-todo-app example with declarative FAST HTML and webui prerendering

[janechu]

Summary

Adds a new examples/webui-todo-app that fulfills the integration-with-webui requirements:

• Declarative syntax: Converts the todo-app to use @microsoft/fast-html declarative templates (<f-template>, <f-repeat>, {{}}/{} bindings) instead of imperative html tagged template literals
• Prerendering via @microsoft/webui: An Express server uses build() at startup to compile the HTML template into a binary protocol, then render() on each request to inject prerendered todo items and component template definitions as raw HTML
• Same functionality as todo-app: add todos, filter (all/active/completed), mark done, remove items

Architecture

Server-side (server.js)

• build({ appDir: './src', plugin: 'fast' }) compiles src/index.html at startup
• Per-request: render(protocol, { prerenderedItems, componentTemplates }) injects:
  • {{{prerenderedItems}}} — raw HTML of todo <li> items, visible before JS loads
  • {{{componentTemplates}}} — <f-template> definitions for client-side hydration
• Injects window.__INITIAL_STATE__ so the client hydrates with the same data

Template (src/index.html)

• <todo-app defer-hydration> with <template shadowrootmode="open"> DSD — prerendered content shows immediately
• {{{prerenderedItems}}} placeholder in the shadow DOM <ul> for webui to fill
• {{{componentTemplates}}} placeholder for <f-template> injection after the element

Client (src/main.ts)

• RenderableFASTElement mixin with templateOptions: "defer-and-hydrate"
• TemplateElement.define({ name: 'f-template' }) registers the template processor
• TodoList.provide(document, new DefaultTodoList(window.__INITIAL_STATE__)) seeds state

Declarative <f-template> bindings (single-brace {}, client-only)

<f-repeat value="{item in todos.filtered}">
  <li class="todo">
    <input type="checkbox" :checked="{item.done}" />
    <span class="description">{item.description}</span>
    <button @click="{$c.parent.todos.remove(item)}" aria-label="Remove item">&times;</button>
  </li>
</f-repeat>