Replaces the deferred stub with a live self-sale-vs-trade-in benefit calculator
(web counterpart to Brusnika p.4): editable «цена в объявлении» + bargaining-%
slider recompute the cost breakdown (торг/риелтор/аренда/юр/реклама) and «Ваша
выгода» in real time. Rates live in a single DEFAULT_OFFER_RATES config
(offer-rates.ts), flagged as illustrative market defaults pending Praktika's
commercial terms — NOT an official offer. tsc clean; themed via brand CSS vars.
Refs #657#658