medcenterstart: apply the v2 design and add the callback dialog

v2 differs from legacy/new only by the callback dialog; the calculator and
the new phone number were already in the React landing.

The header «Звонок» button opens the «Перезвоним вам» dialog from the
fitness landing (name, phone, consent). Unlike fitness, the phone field
keeps this landing's +7 mask and refuses fewer than 11 digits, as the
mockup does. The server accepts form 'callback', names the deal
«Обратный звонок — <имя>» and tags it «обратный звонок». The phone link
inside the dialog and in the section sheet still reports a call click.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

medcenterstart/shared/lead.ts

@@ -1,7 +1,7 @@
 import { z } from 'zod'
 
 /** Which form on the page produced the lead. Surfaces as an amoCRM tag. */
-export const leadFormIds = ['request'] as const
+export const leadFormIds = ['request', 'callback'] as const
 export type LeadFormId = (typeof leadFormIds)[number]
 
 export const utmKeys = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_content', 'utm_term'] as const