Add search functionality

Implement a search interface on the homepage that allows users to find content by keywords, tags, or title. Results should include all categories and display relevance/matching highlights.

Planned

• Search box on homepage
• Real-time search results (or form submission)
• Filter by content type (journal, wiki, research, tasks)
• Highlight matching text in results
• Keyboard shortcut (e.g., Cmd+K)