refactor(frontend): add explicit facet types for search result signal

[SaitejaKommi]

Summary

This PR introduces shared explicit interfaces for search result facets and charts, replacing weak Object typing in SearchResultDetail.

It aligns facet-related types across shared signal state and facet component implementations, improving maintainability and type safety without changing runtime behavior.

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Related Issue

Fixes #388

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What Changed

• Added explicit shared interfaces for search result facets
• Added shared interfaces for chart-related facet data
• Replaced weak Object typing in SearchResultDetail
• Unified facet-related typing across signal state and components
• Improved static type safety and developer ergonomics

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Affected Files

• signals.ts
• search-facets.ts
• search-result-interfaces.ts

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Testing

Full local gate executed successfully:

• Backend unit tests ✅
• Backend integration tests ✅
• Frontend build and tests (dev) ✅
• Frontend build and tests (prod) ✅

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Notes

This is a frontend refactor focused on stronger typing and maintainability with no functional behavior changes.