feat: display retrieval citation highlights

[EDDIWARD]

Summary

Add interactive retrieval citations to both chat experiences.

• Render citation markers by the full source key, preventing collisions between different retrieval tools.
• Open the matching source card and show the retrieved Chunk when a marker is selected.
• Highlight exact lexical retrieval matches in blue and answer–Chunk overlaps in yellow.
• Preserve the citation metadata for streaming responses and restored conversation history.

Why

Users can now see which retrieved Chunk supports a statement, instead of only seeing a long source preview. The same visual behavior is available in both /newchat and the legacy /chat page.

Validation

• Frontend type check passed.
• Frontend production build passed.
• Backend unit tests for the affected conversation and retrieval paths passed.
• Verified locally in both chat pages with knowledge-base retrieval.

UI screenshots are attached in a follow-up comment.

[EDDIWARD]

UI verification

New chat (/newchat) — source 4 is selected; blue marks exact lexical retrieval hits and yellow marks answer–Chunk overlaps.

Legacy chat (/chat) — the same selected source and highlight rules are rendered in the original chat page.

[codecov]

Codecov Report

❌ Patch coverage is 83.01887% with 9 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...dk/nexent/core/tools/knowledge_base_search_tool.py	53.84%	4 Missing and 2 partials ⚠️
backend/services/agent_service.py	33.33%	1 Missing and 1 partial ⚠️
sdk/nexent/vector_database/elasticsearch_core.py	93.33%	0 Missing and 1 partial ⚠️

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