fix(postgres): browse on read-only; page with ORDER BY PK; do not block on COUNT(*)

[ZhuchkaTriplesix]

Summary

Table Browser _connectAndLoad always acquires readWrite (“for custom SQL and REFRESH”). Browse SELECT * … LIMIT n OFFSET m has no ORDER BY, so pages skip/duplicate. COUNT(*) is a sequential scan on large heaps and shares the 30s queryTimeout that forceCloses the pooled socket.

Scope

• Default browse session: PgSessionMode.readOnly (Save / REFRESH / custom SQL may upgrade — see shared-socket issue).
• When a PK exists, ORDER BY those columns for stable pages.
• Do not require a blocking COUNT(*) before first paint: estimate (reltuples) or count asynchronously; timeout must not kill the browse socket.

Out of scope

• Changing default page size (200).

[ZhuchkaTriplesix]

Shipped in #859