fix(search): support exact MXID people search and fallback to full list on empty local results

[tyreseluo]

Summary

• Improve remote People search by supporting exact MXID lookup (@user:server) via profile fetch.
• Merge exact-lookup and user-directory search results with de-duplication by user ID.
• Add logging for remote directory search requests to improve diagnosability.
• Improve UX for local filtering:
  • When room search has no matches, fallback to the same full list shown for empty search input.
  • When space search has no matches, fallback to the same full list shown for empty search input.

Why

• Some homeserver directory behaviors do not reliably return results for full MXID queries.
• Empty local search results felt like a dead end; fallback keeps navigation context and reduces friction.

Testing

• cargo check -q passes.
• Manually verified:
  • People search can find full MXID targets.
  • Non-empty keywords with zero local matches now show full rooms/spaces list.