ui(sql): surface open/save file errors in workspaces

[ZhuchkaTriplesix]

Parent

Part of #445

Severity

Medium

Problem

Open/save SQL file failures are catch (_) {} — silent no-op. User thinks save succeeded or open did nothing.

Evidence

• lib/features/mysql/mysql_sql_workspace.dart (~287, ~302) and postgres/sqlite/extension SQL workspaces
• Contrast: export save error dialog in results_tab.dart

Acceptance

• Failed open shows user-visible error (dialog or toast)
• Failed save shows user-visible error
• Success path unchanged

Suggested fix

Same pattern as ResultsTab export error dialog / shared snackbar helper.

[ZhuchkaTriplesix]

Shipped on dev via #462 (Closes did not auto-close because PRs targeted dev).