ui(connections): show tree load errors with Retry (PG/MySQL)

[ZhuchkaTriplesix]

Parent

Part of #445

Severity

Medium

Problem

PG/MySQL connection tree load failures only clear _loading — no _error UI. Failed expand looks like an empty schemas/objects tree. Extension tree already shows error + Retry.

Evidence

• lib/features/connections/connections_panel_pg_tree.dart (~314–316, ~646–648)
• lib/features/connections/connections_panel_mysql.dart (~416–418)
• Good pattern: connections_panel_extension.dart (~289–311)

Acceptance

• Failed expand/load shows inline error message
• Retry action reloads the failed node/branch
• Empty successful load still shows empty-state copy (not error)

Suggested fix

Store error string per panel/node; reuse extension tree error row UI.

[ZhuchkaTriplesix]

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