fix(ui): tree errors, empty table state, scrollable toolbars

[ZhuchkaTriplesix]

Summary

• ui(connections): show tree load errors with Retry (PG/MySQL) #449 PG/MySQL connection tree: inline error + Retry on failed expand/load
• ui(table): empty-state when browse returns no columns #450 MySQL/Postgres/SQLite table browse: empty-state when no columns
• ui(mysql): horizontal scroll for table toolbar under narrow width #451 MySQL (+ SQLite) table toolbar: horizontal scroll under narrow width

Closes #449
Closes #450
Closes #451

Test plan

• Fail PG/MySQL tree load — error + Retry, not empty list
• Browse empty/broken table — “No columns returned” + Retry
• Narrow MySQL table pane — no RenderFlex overflow on toolbar