Assessment UI: Streamlined design improvements #289

Is your feature request related to a problem?
The current Assessment UI is fragmented, splitting functions across three tabs and a 5-step wizard. This leads to inefficiencies in accessing saved configurations, understanding input/output relationships, and maintaining stable column orders.

Describe the solution you'd like
Rebuild the Assessment UI with two views on the BATCH assessments API:

• Home: Assessor and Runs panels for searching, version management, result viewing, and CSV export.
• Wizard: A streamlined 4-step flow with options for new, edit, and existing assessments.
• Prompt editor: Enhanced editing features with live previews, token highlighting, and user-friendly interactions.
• Results: Stable output display joined with source rows, maintaining column order and caching for performance.
• Credentials: Integration with Google Vertex AI for easier setup.
• BFF routes: Implementing specific API endpoints for improved functionality.
• Implement tab-polling control based on visibility.