Add validation

- Most fields are required unless they have an optional field
- "let us know" fields & Add explanation textareas are optional.

- [ ] If a user skips a required field, show invalid message and coloring on field ([see VA example](https://www.vets.gov/education/apply-for-education-benefits/application))
- [ ] If a user clicks next and no choice has been selected within a radio set, show error & refocus to field

Fields to validate:
- [ ] email address
- [ ] phone number (also autoformat the phone number to include dashes)
- [ ] BONUS: autofill address (Google Maps API?) (@juliazbyron: research examples of good address autofilling)
- [ ] zip, make sure it has 5 numbers or 9 (with dash?)

Start with the validation checklist in this issue and review the linked VA example for the expected invalid styling and focus behavior. Confirm how required fields, radio sets, email, phone, and ZIP inputs should behave, including phone formatting; the optional Google Maps address autofill is separately marked as a bonus. Done means all listed validation cases work and optional fields remain optional.

Written by the indexing model from the issue text.