Handle Schwab interest withholding taxes

[lgyanf]

This PR fixes issue #705

Summary

• Treat Schwab NRA Tax Adj rows without symbols and with SCHWAB1 INT as account-level interest tax.
• Report interest tax totals as positive values and surface them in text/PDF outputs.
• Add regression tests for interest tax handling and update expected outputs.

Testing

• uv run pytest
• uv run pytest tests/general/test_calc.py::test_interest_tax_totals_are_positive
• uv run pytest tests/schwab/test_schwab_interest_tax.py

[vmartinv]

I think this is safe, I'm a bit curious on the motivation. Schwab has recently sent an email saying:

Effective January 2026, the Schwab One® Interest feature on your account is no longer subject to U.S. tax withholding. To be exempt from U.S. tax withholding, you must have a current, valid Form W-8BEN on file with Schwab.

Which will make this PR not very useful at least for this particular fund from now on. On top of paying foreign tax, the interest paid is also very low, I never received enough to justify worrying about these transactions.
It could be useful for other funds and to get rid of errors though, so worth merging.

[vmartinv]

After merging, looks like the new test needs fixing

[vmartinv]

@lgyanf any chance you can rebase and fix this ?