Validate: reject phone numbers that fail libphonenumber

[1nonlypiece]

Summary

The profile settings phone field (components/settings/ProfileSection.tsx) autosaved on every keystroke with no validation at all. Adds isValidProfilePhone() (lib/validation/phone.ts, wrapping libphonenumber-js) and wires it into handlePhoneChange: an invalid number shows an inline role="alert" error and skips triggerSave(); a valid one clears the error and autosaves as before. An empty/whitespace value is treated as valid since the field isn't required. Requires an international (+-prefixed) number since there's no country selector to supply a default region.

Also adds the new settings.profile.phone_invalid translation key to both en.json/es.json -- the settings.* tree is otherwise entirely missing from the locale files (a large pre-existing gap, out of scope to backfill here), but I didn't want to add another broken key on top of it.

Testing

• node node_modules/vitest/vitest.mjs run ... tests/unit/validation/phone.test.ts tests/unit/settings/ProfileSection.test.tsx -- 8/8 pass (pure validation function edge cases, plus the component showing/clearing the error and not erroring on the valid initial value)
• npm run test:unit:vitest -- 305 passing (was 297 on main), zero regressions
• npx eslint on all touched files -- clean
• npx tsc --noEmit -- no new errors in any touched file

Closes #1470