feat: add Zendesk Help Center embed widget (manual setup)

[keshavk-msft]

Summary

Adds a Zendesk Help Center embed sample under ui/embed/zendesk-widget/.
Embeds a Copilot Studio agent as a floating chat widget in a Zendesk Help Center theme using manual setup (theme code editor).

What's included

• Complete manual setup guide (Zendesk theme code editor — no deploy script required)
• Same core bundle (copilot-chat.js) as the ServiceNow widget sample
• MSAL authentication (silent SSO → popup fallback)
• WebChat rendering with customizable styling
• Local test page for development
• Updated ui/README.md and ui/embed/README.md with zendesk-widget entry

Relationship to servicenow-widget

The core source files (src/) and build config are identical to ui/embed/servicenow-widget,
with one enhancement: auth.ts adds a 2s timeout to ssoSilent() to preserve the
browser's user-gesture window for the popup fallback.

The deployment method differs — Zendesk uses theme template injection
(document_head.hbs + footer.hbs) instead of ServiceNow Widget Dependencies.

Files changed

• ui/embed/zendesk-widget/ — new sample (17 files)
• ui/README.md — added zendesk-widget to embed table
• ui/embed/README.md — added zendesk-widget to samples list