feat(tax): VAT rates by country/state with a default, per-line VAT, invoicing catalog

[hedon77]

Polish VAT setup: rates matched by country/state with an is_default fallback, a per-line VAT rate with descriptive labels, and an invoicing catalog (products with a unit field). Removes the secondary tax (tax2) and the tax levels.

[Panelica]

Big, well-tested piece — thank you. Two things to confirm before we merge, since it's the foundation the other invoicing PRs build on:

1. It removes secondary tax (tax2) entirely. The deleted test shows it never actually charged, so we're fine removing dead behavior — just confirming this is the intended scope (multi-tax jurisdictions would rely on the new per-line rates instead).
2. On existing installs the migration leaves is_default = false on every rule, so the global-default fallback returns nothing until an operator marks a default — a catch-all tax could stop applying after upgrade. Could we set a sensible default in the migration (e.g. promote the former catch-all), or note it in the upgrade steps?

We'll take this as the base and rebase #9/#15/#17 on top to avoid the invoice-file conflicts.

[hedon77]

Confirmed on both points:

1. Removing ax2 is intended — the deleted test showed it never actually charged, so it was dead behavior. Multi-tax jurisdictions rely on the new per-line rates instead.
2. Good catch — the migration now promotes the old catch-all rule (empty country) to is_default = true (falling back to the first rule if none exists), so the global default keeps applying after upgrade.