refactor: replace custom sidebar search with native .searchable()

[datlechin]

Summary

• Replace the custom TextField-based search field in SidebarView with SwiftUI's native .searchable() modifier on the NavigationSplitView sidebar column
• Remove localSearchText state, searchSyncTask debounce task, and the entire searchField computed property (~55 lines removed)
• Move the .searchable() binding to ContentView via sidebarSearchTextBinding(for:) so it applies at the NavigationSplitView level
• Simplify onChange(of: sidebarState.searchText) to directly update viewModel.debouncedSearchText

Test plan

• Open a connection with tables — verify the search field appears in the sidebar toolbar area
• Type to filter tables — verify filtering works
• Clear search — verify all tables reappear
• Verify search state syncs across native tabs for the same connection

[chatgpt-codex-connector]

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