### Summary
This PR trims the Tax ID value on the frontend so that whitespace-only
entries (e.g. " ") are correctly treated as "not set" instead of a valid
value.
Previously the "is the Tax ID set?" checks relied on string
length/truthiness, so whitespace-only input slipped through as a real
value - bypassing validation and getting persisted.
### Testing
- Subscription upgrade & top-up forms: verified the form no longer
submits when only whitespace is entered for the Tax ID. The validation
message now shows.
- Billing address form: no inline message is shown, but the Tax ID is
submitted as null, which clears it. This is unchanged from current
behaviour.
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **Bug Fixes**
* Tax ID values are now trimmed before validation and submission,
reducing errors caused by accidental leading or trailing spaces.
* Billing and payment flows now consistently use the cleaned Tax ID
value when deciding whether to save or send it.
* Customer tax ID details passed to payment setup now reflect the
trimmed value, helping avoid mismatches with external payment
processing.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
Kanishk Dudeja
1f03c0dc0e