ui(stats/table): clear loading on early exit and unchanged replaceIfChanged

[ZhuchkaTriplesix]

Parent

Part of #445

Severity

High

Problem

1. replaceIfChanged early-return skips _loading = false in MySQL/Postgres stats → stuck “Connecting…” when stats are identical/null while loading. Extension/Redis already guard (if (_loading) setState(() => _loading = false)).
2. After _loading = true, _fetch does if (conn == null || !conn.isConnected) return with no error / loading clear → infinite spinner in stats and table views.

Evidence

• lib/features/mysql/mysql_stats_view.dart (~95–102, 111–114)
• lib/features/postgresql/postgres_stats_view.dart (~111–114)
• Table views: mysql_table_view.dart, postgres_table_view.dart, sqlite_table_view.dart early conn checks
• Good pattern: lib/features/extensions/extension_stats_view.dart (~84–86)

Acceptance

• Unchanged replaceIfChanged still clears _loading on first paint
• Null/disconnected lease sets _error (or equivalent) and _loading = false
• Tests covering both paths for at least one stats view

Suggested fix

Match extension_stats / redis_view guards; treat missing connection as connect failure.

[ZhuchkaTriplesix]

Shipped on dev via #460 (Closes did not auto-close because PRs targeted dev).