Display token counting statistics in chat UI

[manavgup]

Priority

🔴 High

Description

Add visual display of token usage (prompt tokens, completion tokens, total) for each chat response.

Current State

Backend tracks tokens but UI doesn't display them

UI Evidence

Tested "Token Stats Test Session" - no token stats visible in responses

Files to Modify

• frontend/src/components/search/MessageDisplay.tsx
• frontend/src/types/search.ts
• frontend/src/components/search/TokenStats.tsx (new)

Acceptance Criteria

• Display prompt tokens, completion tokens, total tokens per message
• Show token costs if available
• Add tooltip with breakdown
• Color-code high token usage (yellow/red warnings)
• Add toggle to show/hide token stats

Effort

3-5 days

Related Issues

#253, #236