Default tax rates calculate correctly on new estimates

AI-generated estimates now display the correct tax amount immediately instead of showing $0.00.

Bug Fixes

AI-generated estimates were displaying $0.00 for default tax lines until you manually removed and re-added the tax rate. This is now fixed — tax rates show the correct calculated amount as soon as the estimate is created.

Improvements

The tax rate selection has been refined with a searchable dropdown (Combobox) for faster selection, and adding or removing tax rates now shows visual feedback with spinners so you know the action is in progress.