Add virtual pages from Shopify and panel refresh button

This simplifies product management by eliminating manual Kirby page creation. Products are now automatically loaded as virtual pages from the Shopify API.

Changes:
- Add virtual pages via page.children:after hook
- Create shopify.php helper with caching (60min TTL)
- Add shopify-refresh panel plugin for cache management
- Remove manual product content files (now virtual)
- Update site.yml blueprint to show refresh button
- Fix cache implementation to use get/set pattern

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

site/plugins/shopify-refresh-button/.gitattributes

@@ -0,0 +1,11 @@
+# Note: You need to uncomment the lines you want to use; the other lines can be deleted
+
+# Git
+# .gitattributes export-ignore
+# .gitignore export-ignore
+
+# Tests
+# /.coveralls.yml export-ignore
+# /.travis.yml export-ignore
+# /phpunit.xml.dist export-ignore
+# /tests/ export-ignore