Add AI generation to the wizard and embed Playground

[akirk]

Summary

• New Build with AI section in the GitHub Pages wizard: describe the app, pick Anthropic / OpenAI / Local (Ollama, LM Studio), enter an API key (stored in the browser only, requests go straight to the provider). The model edits the scaffold through list_files / read_file / write_file / edit_file / delete_file tools over an in-memory file map, with streaming output and a file viewer; follow-up prompts refine the same plugin. The wp-app README is pulled into the system prompt.
• Run in Playground now embeds Playground in an iframe and installs the plugin zip as a literal resource (same approach as Plugin Sumission: Provide a button to let the user plugin check before submitting WordPress/wordpress.org#718), so a generated app is not limited by the blueprint URL length. Download and Playground both use the AI-edited files.
• The AI Assistant checkbox is removed: an embedded Playground fetches resources from our origin and cannot reach GitHub.
• pages.yml unchanged apart from nothing new to bundle; wp-app.zip is now used by both download and Playground.

Test plan

• Headless Chromium with a mocked Anthropic SSE stream: tool calls (incl. an error result), streamed text, file list, settings persistence per provider, then a real embedded Playground boot that lands on the app URL with the generated file installed, no console errors.
• Real Anthropic / OpenAI / local model run (no key available here).