fix: persist and restore PostgreSQL schema on reconnect, fix stuck error dialog

[datlechin]

Summary

Fixes #540 — two critical bugs when reconnecting to PostgreSQL with a non-default schema.

• Schema lost on restart: schemaName was never persisted in tabs or UserDefaults. After relaunch, the restored query omitted the schema prefix (SELECT * FROM "table" instead of SELECT * FROM "schema"."table"), causing relation does not exist errors. Now persisted through PersistedTab, QueryTab, EditorTabPayload, and AppSettingsStorage, and restored on reconnect via DatabaseManager + restoreSchemaAndRunQuery.
• Stuck error dialog: Alert sheets used NSApp.keyWindow which could return a SwiftUI sheet window. beginSheetModal(for: sheetWindow) creates an undismissable sheet-on-sheet. Now uses a stable contentWindow reference stored on the coordinator.

Test plan

• Connect to PostgreSQL → switch to non-public schema → open table → quit → relaunch → reconnect → verify table loads with schema-qualified query
• Trigger a query error (e.g. invalid SQL) → verify OK button dismisses the error dialog
• Trigger a query error while a .sheet() is open (e.g. database switcher via Cmd+D) → verify dialog still dismisses
• Open multiple tabs across different schemas → quit → relaunch → verify all restore correctly
• Verify backward compatibility: old persisted tab JSON (without schemaName) loads without errors
• FK navigation on non-public schema tables works correctly
• Preview tab mode on non-public schema tables works correctly