Make search results human-readable

[princerm06]

Summary

• replace similarity/debug metadata with recognizable source domains and grounded result context
• make result titles and Open original actions clearly navigable
• add a plain-language Why this matched section backed by the retrieved excerpt rather than generated explanation
• improve search loading, empty, and error states
• polish the search page layout and mobile behavior

Product intent

This is Milestone 4D: keep chunk-level retrieval under the hood while presenting results in a way a nontechnical user can immediately understand: what the page is, where it came from, why it matched, and how to open it.

Scope

No retrieval, embedding, database, schema, or deployment behavior changes. Similarity scores remain available in the API but are no longer exposed in the normal search UI.

Verification

Backend CI should remain green. The remaining verification is a local browser pass against the running Synapse backend to confirm result rendering, source labels, loading/error states, and outbound links.