feat: redesign copy page button as multi-format dropdown menu

Redesigned the Copy page button into a dropdown menu with multiple
copy formats and AI service integrations:

Copy formats:
- Copy as Markdown: formatted for LLMs and AI tools (uses Turndown)
- Copy as Plain Text: unformatted text content
- Copy Page URL: share the page link with title

AI integrations:
- Open in ChatGPT: sends page URL with analysis prompt
- Open in Claude: sends page URL with analysis prompt
- Open in Perplexity: sends page URL for research

Design features:
- Dropdown menu with icons, labels and descriptions
- Visual 'Copied!' feedback with green checkmark
- Click-outside and Escape key to close
- Responsive: icon-only on small screens
- Full i18n support (en, es, zh, ja)

Author: cursoragent