Turn StateSelect and CitySelect into autocomplete comboboxes so long
lists no longer bias people toward the top option. A shared FormCombobox
wraps shadcn's (Base UI) Combobox and bridges it to react-hook-form via
useController, storing the state code / city name while displaying a
label. Matching is accent-insensitive and prefix-based; the list opens
after 1 character and results are alphabetical. State searches by both
full name and initials (new StateNames map).
Adds @base-ui/react + shadcn combobox/input-group primitives, and
documents our export-const and named-hook-import conventions in the
components rule (with the src/components/ui/* shadcn carve-out).
