feat: add chat retrieval controls and summary

[mtsui-cmyk]

Description

Improve chat retrieval transparency and control.

This PR adds a retrieval summary to assistant responses and introduces explicit retrieval modes for chat when knowledge bases are selected.

Changes

• Show retrieval summary for chat responses:
  • knowledge base name
  • matched file count
  • retrieved chunk count
• Add chat retrieval modes:
  • Auto
  • KB
  • KB + Web
  • Off
• Keep the implementation frontend-scoped by using existing metadata and request snapshot overrides

Related Issues

• Improves chat retrieval visibility and control

Module(s) Affected

• web (Frontend)

Checklist

• I have read and followed the contribution guidelines.
• My code follows the project's coding standards.
• I have run pre-commit run --all-files and fixed any issues.
• I have run relevant validation for the touched files.
• I have added relevant tests for my changes.
• I have updated the documentation (if necessary).
• My changes do not introduce any new security vulnerabilities.

Validation

• node ./scripts/run-node-tests.mjs tests/retrieval-summary.test.ts tests/chat-retrieval-mode.test.ts
• npx eslint app/'(workspace)'/chat/'[[...sessionId]]'/page.tsx components/chat/home/ChatComposer.tsx components/chat/home/ChatMessages.tsx lib/retrieval-summary.ts lib/chat-retrieval-mode.ts tests/retrieval-summary.test.ts tests/chat-retrieval-mode.test.ts