Text Selection for Context Addition

[abdeliibrahim]

Technical Tasks:

1. Implement text selection functionality in iframe PDF viewer (may require PDF.js integration)
2. Create context addition modal/panel for managing selected text
3. Add "Add to Context" button for selected text
4. Update chat API to accept additional manual context
5. Create context management UI component showing all added selections
6. Implement context snippet removal and editing
7. Add manual context to vector search alongside automatic retrieval