Document the pre-chat form #70

The widget now has a configurable pre-chat form that collects details like name and email before the conversation starts. It shipped in @chatcops/widget 0.4.0, but the docs site only mentions it in the request schema on the Server Setup page. The WidgetConfig reference on the Configuration page also leaves out preChatForm, onLeadCaptured and onPreChatSubmit.

Where to look

• packages/widget/src/widget.ts (PreChatField, PreChatFormConfig, WidgetConfig, handlePreChatSubmit, showChatView)
• packages/widget/src/dom/prechat-form.ts (field types and validation)
• packages/widget/dev.html (a working example config)
• website/src/content/docs/widget/configuration.mdx
• website/astro.config.mjs (sidebar)

Acceptance criteria

• New page website/src/content/docs/widget/pre-chat-form.mdx, added to the Widget section of the sidebar
• Documents enabled, title, subtitle, submitLabel, and each field option: name, type (text, email, select or textarea), label, placeholder, required, options
• Explains how the form behaves:
  • it only shows when there is no saved conversation and it hasn't been submitted in this browser session
  • required fields and email format are validated
  • a field named name personalizes the welcome message
  • a field named message is sent as the first chat message
  • answers go to the server as userData (up to 50 fields, names up to 100 characters, each value up to 500 characters) with the next chat request, are re-sent if that request fails, and the server adds them to the system prompt for that request
• Documents onPreChatSubmit and the preChatSubmit event
• Notes that the form can only be configured from JavaScript (there are no data-* attributes for it)
• WidgetConfig in configuration.mdx includes preChatForm, onLeadCaptured and onPreChatSubmit
• pnpm --filter @chatcops/website build succeeds
• Docs only, no changeset needed

Want to work on this? Comment below and I'll assign it to you. Please take one issue at a time, branch off dev, and open your PR against dev with Closes #<this issue number>. Setup notes (pnpm, build before test) are in CONTRIBUTING.md.