feat: add WB API tax-calculator web app (Express backend + frontend)

[minislon]

Motivation

• Provide a deployable web application that pulls sales reports from Wildberries and produces preliminary tax calculations for a selected period.
• Replace the previous static toy app with a small server + UI so the WB token can be used immediately and results are aggregated server-side.
• Keep integration practical while noting that the current WB report endpoint is deprecated and should be migrated to the newer finance endpoints in a follow-up change.

Description

• Added an Express backend (server.js) exposing POST /api/wb-tax/calculate which calls the WB report endpoint, aggregates rows and computes tax for two regimes (УСН Доходы 6% with insurance reduction, and УСН Доходы-Расходы 15% with 1% minimum).
• Added frontend under public/ (index.html, app.js, style.css) with a form to submit WB token, date range and parameters and to display the computed results and raw JSON details.
• Project config and packaging updates: new package.json (start/dev/check scripts, express dependency), generated package-lock.json, and a basic .gitignore.
• Updated README.md with run and deploy instructions (local run with npm install && npm start, and recommended platforms such as Render), and a note about the deprecated WB endpoint and suggested migration targets.

Testing

• Installed dependencies with npm install which completed successfully.
• Syntax checks were run with npm run check (node --check server.js && node --check public/app.js) and passed.
• Basic runtime smoke test started the server and fetched the homepage using node server.js + curl http://localhost:3000 which returned the frontend HTML. All automated checks succeeded.

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Codex Task

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Remove percentage field from commission expense sum

The expense aggregation mixes units by adding commission_percent together with ruble-denominated fields; when this endpoint returns a percent value per row (as the field name indicates), the code inflates wbCommission and therefore understates tax in the usn_income_expenses regime. Only monetary commission fields should be included in expense totals.

Useful? React with 👍 / 👎.

[chatgpt-codex-connector]

Avoid summing two sale amount fields per row

The grossSales calculation adds both retail_amount and retail_price_withdisc_rub for every record, so any WB response where both keys are populated will count the same sale twice and overstate income, which directly skews tax output for both supported regimes.

Useful? React with 👍 / 👎.