v2.1.0: Bookmarks header, TTS cache, clickable source links, Redash export, starter translations

New features:
- Bookmark button in header with dropdown panel (moved from settings)
- TTS audio caching (eliminates repeat API calls for same message)
- Clickable source attribution pills with hover preview
- SSE metadata event for page/course URLs
- Redash analytics export endpoint with API key auth
- Translated conversation starter prompts (26 languages)
- Language selector clarified as "SOLA Language" with description

Bug fixes:
- SOURCE tag regex now handles whitespace variations
- More permissive bracket stripping prevents tag leaks

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: caswelltom