docs(web): define contact phone input

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: AchoArnold

docs/superpowers/specs/2026-08-21-contact-phone-input-design.md

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+# Contact Phone Input Design
+
+## Goal
+
+Use the existing `v-phone-input` component for phone numbers in the contact
+add/edit dialog.
+
+## Design
+
+Represent each form phone row with a phone number value and its own country.
+Bind those fields to `v-phone-input` using `v-model` and `v-model:country`,
+following the messages page configuration.
+
+Existing E.164 phone numbers derive their country with `libphonenumber-js`.
+Numbers without a detectable country and newly added rows default to `US`.
+Removing a row removes both its number and country because they are stored in
+one object.
+
+## Behavior
+
+- Keep support for multiple phone numbers.
+- Preserve current required-phone validation and server error display.
+- Submit only trimmed, non-empty phone number values to the API.
+- Keep add, edit, and remove behavior unchanged apart from the country-aware
+  input.
+
+## Validation
+
+Run ESLint, Stylelint, and Prettier against the changed contact page.