feat(ui): upgrade ExtensionTableView with TableToolbar, custom SQL filter, and async count calculation

[ZhuchkaTriplesix]

Summary

Upgrades ExtensionTableView with the universal ExtensionTableToolbar, custom SQL filtering (WHERE clause), asynchronous row count calculation (count()), and DDL inspection support.

Closes #324
Part of Epic #322

Changes

• Created ExtensionTableToolbar (lib/features/extensions/extension_table_toolbar.dart) following the design of PostgresTableToolbar. Includes:
  • Title and table/view icon with responsive horizontal scroll handling on narrow windows.
  • Pagination and status chip (paginationLabel).
  • DDL Inspection button: opens modal dialog displaying table structure fetched via db.getObjectMetadata.
  • Filter button: toggles SQL WHERE clause text field.
  • Refresh, Back, Next buttons.
• Upgraded ExtensionTableView (lib/features/extensions/extension_table_view.dart):
  • Asynchronous count calculation: SELECT count(*)/count() runs asynchronously in the background (_fetchCountAsync) after the initial row data page (SELECT * ... LIMIT/OFFSET) loads, eliminating UI freezing on large datasets.
  • Custom SQL filter: users can enter dynamic WHERE filters (e.g., id > 100 AND status = 'active') which immediately apply to both page queries and count calculations.

Verification

• Ran flutter test test/features/extensions/extension_table_toolbar_test.dart (All tests passed).
• Ran flutter analyze across project (No issues found).
• Ran full regression test suite flutter test (All 813 tests passed).