ui(table): empty-state when browse returns no columns

[ZhuchkaTriplesix]

Parent

Part of #445

Severity

Medium

Problem

Success with empty _columnNames renders bare Container(color: background) — blank workspace, no empty copy, no Retry.

Evidence

• lib/features/mysql/mysql_table_view.dart (~407–408)
• lib/features/postgresql/postgres_table_view.dart (~449–450)
• lib/features/sqlite/sqlite_table_view.dart (~394–395)

Acceptance

• Empty columns/data shows readable empty state (“No columns / no data”)
• Refresh / Retry action available
• Error path still distinct from empty success

Suggested fix

Shared empty-state widget used by all three table views.

[ZhuchkaTriplesix]

Shipped on dev via #461 (Closes did not auto-close because PRs targeted dev).