feat: add Tailcall Config Generator UI with dynamic schema-driven form

[visq97]

Summary

This PR adds a Tailcall Config Generator UI — a visual, schema-driven form for generating valid Tailcall Runtime Configuration files.

What's New

• /config-generator — standalone page with the full config generator
• /playground — now has two tabs: "GraphQL Playground" (existing) and "Config Generator" (new)
• /app/config — redirects to /config-generator

Features

• Dynamically loads the official Tailcall config schema from tailcallhq/tailcall main branch
• Tabbed interface: Server, Upstream, Telemetry, Links
• Searchable dropdowns for enum fields (HTTP version, link types, telemetry exporters, etc.)
• Toggle switches for boolean fields
• Number inputs with min/max validation
• Nested object support (e.g., batch settings, headers, CORS)
• Array support with add/remove (e.g., links, allowed headers)
• Key-value pair editor for free-form objects (e.g., vars, custom headers)
• Live preview of generated config in the right panel
• Output in YAML or JSON format
• Download as .yml or .json file
• Copy to clipboard button
• Reset to defaults
• Dark theme matching Tailcall's design system
• Fully responsive

Technical Details

• Built with React 18 + TypeScript (matches existing stack)
• Uses react-select for searchable dropdowns
• Uses js-yaml for YAML serialization
• Uses file-saver for download functionality
• No heavy form libraries — custom lightweight form renderer for full control over UX
• Schema is fetched at runtime from the official Tailcall repo

Testing

• Build passes: npx docusaurus build ✅
• TypeScript: no errors in new code ✅

/claim #373