refactor(workspace): modularize shared SQL editor and result workbench components

[ZhuchkaTriplesix]

Motivation & Problem

Currently, lib/features/main_screen/ acts both as the main window container (MainScreen, ShellChrome, ConnectionsPanel) and as a shared library of workbench components used by all database workspaces (mysql/, postgresql/, sqlite/, extensions/).

Specifically:

• query_editor_tab.dart
• results_tab.dart
• sql_editor_chrome.dart
• destructive_query_dialog.dart
• dml_preview_dialog.dart
• data_grid_staging_buffer.dart
• sql_query_history_dialog.dart

Proposed Scope

1. Reorganize and modularize workbench components:
   • Group shared SQL editor, result grid, and DML staging components into a dedicated module (lib/features/workspace/ or lib/shared/workbench/).
   • Keep main_screen.dart focused strictly on window frame layout and top-level panel coordination.
2. Update feature workspace imports across PostgreSQL, MySQL, SQLite, and Extensions.
3. Ensure zero regressions across widget and integration tests.

Acceptance Criteria

• Reusable workspace components have a dedicated, cohesive home.
• main_screen is decoupled from engine-specific workspace assembly.
• All unit and widget tests pass.