Allow users to select individual transactions for import instead of
automatically importing all non-duplicate transactions. Adds checkboxes
to the preview table with select-all functionality and updates the
import logic to only process selected transactions.
<img width="1076" height="592" alt="image"
src="https://github.com/user-attachments/assets/06dab7d0-87f1-4c69-97a4-fa98b33062d7"
/>
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