Fix names: replace 500 ms polling timer with push-based debounce

[ivandrofly]

Summary

• Replace the System.Timers.Timer polling loop in FixNamesViewModel with a push-based debounce (CancellationTokenSource + Task.Delay). The old timer fired every 500 ms for the entire lifetime of the window, re-joining the checked names into a string on each tick just to detect whether anything changed — even while the user did nothing. Preview generation is now only scheduled when a checkbox actually changes, and rapid toggles collapse into a single regeneration 500 ms after the last change.
• Checkbox cell template now uses a two-way IsChecked binding with UpdateSourceTrigger.PropertyChanged and invokes FixNameItemChangedCommand; the space-bar multi-toggle helper also requests a preview.
• Select all / invert selection / add extra name regenerate the preview directly, removing the _loading / _oldNames / lock bookkeeping the polling approach required.
• Minor cleanup in TableViewExtras.AddSpaceToggle (parameter rename, lambda simplification).

Test plan

• Open Tools → Change casing → Fix names with a subtitle containing names; verify the hits preview populates on open
• Toggle a single name checkbox; verify the preview refreshes ~500 ms later
• Rapidly toggle several checkboxes; verify only one refresh happens after the last click
• Use Select all and Invert selection from the context menu; verify the preview updates
• Select multiple rows and press Space; verify all checkboxes toggle and the preview refreshes
• Add an extra name; verify it is picked up and the preview updates
• Toggle a checkbox and immediately close the window; verify no exception from the pending preview

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