fix(grid): distinguish schema load failure from missing PK

[ZhuchkaTriplesix]

Summary

• Table Browser no longer treats a failed getTableSchema (permissions, disconnect) as “Cannot edit: no primary key detected”.
• Status shows “schema unavailable” plus the real error; editing stays off. Refresh retries schema.
• Genuine missing PKs keep the old copy. SQLite implicit rowid is applied only after a successful schema load.

Closes #772

Test plan

• Force a schema load failure (revoke metadata permissions / disconnect mid-open) → status is “schema unavailable”, not “no primary key”; cells stay read-only
• Refresh after the failure recovers → schema loads, editing follows the real PK (or implicit rowid on SQLite)
• Table with no PK still shows “no primary key detected”
• Views / custom SQL stay read-only with their existing status copy
• flutter test test/features/workspace/table_view_staging_test.dart