feat: add DIFM conversational chat interface

[louwie17]

Summary

• Adds a Bring Your Own Key AI chat assistant under WooCommerce → Hey Woo! — merchants paste an Anthropic API key in Settings and get a conversational interface without needing Claude Desktop or MCP
• On each chat message, PHP fetches a 1-hour-cached store context snapshot (revenue, orders, refunds, customers, top-10 products) via the existing analytics abilities and injects it into Claude's system prompt — so answers are grounded in real store data rather than hallucinated
• The UI is a full-height React chat built with @wordpress/scripts (webpack), rendering inside the standard WooCommerce admin chrome

What's included

PHP

• includes/difm/class-anthropic-client.php — thin wp_remote_post wrapper; constant-first key priority chain (HEY_WOO_ANTHROPIC_KEY > wp_options); validate_key() called on settings save
• includes/difm/class-difm-rest-controller.php — POST /hey-woo/v1/difm/chat + POST /hey-woo/v1/difm/key/validate; store context fetched and cached per-day
• includes/difm/class-difm-admin-page.php — WooCommerce submenu page that enqueues the React bundle via wp_enqueue_script()
• includes/settings/class-settings-page.php — new DIFM section with password field for the API key

TypeScript / React (packages/today/src/)

• useChat.ts hook — manages message history, sending state, and REST calls
• App.tsx — chat layout: header, scrollable role="log" message list, typing indicator, inline error bar
• ChatBubble, ChatInput, NoKey, Loading, Error components
• style.scss — scoped chat styles (.hey-woo-page, .hey-woo-bubble, .hey-woo-typing-indicator, etc.)

Tests

• tests/integration/test-anthropic-client.php — key priority chain, validate_key() with mocked HTTP
• tests/integration/test-difm-rest-controller.php — route registration, permission checks, no_key response, chat reply flow

Test plan

• Add an Anthropic API key under WooCommerce → Settings → Hey Woo — should validate on save and show a success notice
• Navigate to WooCommerce → Hey Woo! — chat interface loads inside normal WC admin chrome
• Ask "what are my top products this month?" — Claude should answer with real product names and revenue figures from the store
• Ask "what were my sales last week?" — should cite actual revenue numbers, not say data is unavailable
• Leave API key empty — navigating to the page should show the NoKey setup prompt with a link to Settings
• Run ./bin/check — 274 tests pass, PHPCS clean

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