Add WebFormsForm component for interactive-mode Request.Form support

[csharpfritz]

Summary

The Request.Form shim (PR #532) currently only works in SSR mode because interactive Blazor Server has no HTTP POST all interactions flow through SignalR. This means Request.Form["field"] returns empty in interactive mode by design.

Proposal

Create a <WebFormsForm> Blazor component that bridges form submissions to the FormShim via JavaScript interop, enabling Request.Form to work in both SSR and interactive modes.

How it works

Mode	Flow
SSR	form POST -> IFormCollection -> FormShim (works today)
Interactive	WebFormsForm -> JS interop reads FormData -> SignalR -> populates FormShim (new)

Implementation approach

1. WebFormsForm component wraps a <form> element
2. JS interop reads FormData from the DOM and sends key/value pairs to C#
3. FormShim.SetFormData() populates the shim from JS interop data
4. WebFormsPageBase integration wires the form to Request.Form

Migration story

<!-- Web Forms -->
<form runat="server">
    <asp:TextBox ID="txtName" runat="server" />
</form>

<!-- Blazor with BWFC -->
<WebFormsForm>
    <input type="text" name="txtName" />
</WebFormsForm>

@@code {
    // Same code works in both SSR and interactive mode
    string name = Request.Form["txtName"];
}

Acceptance criteria

• <WebFormsForm> component renders a <form> element
• JS interop reads FormData on submit and sends to server
• Request.Form["field"] returns submitted values in interactive mode
• Request.Form.GetValues("field") works for multi-value fields
• Component works in both SSR and interactive render modes
• bUnit tests for the component
• Playwright integration tests
• Sample page demo
• Documentation update

Dependencies

• PR feat: Request.Form shim for Web Forms form POST migration #532 (Request.Form shim) must merge first