Date: 2026-01-28 00:00 Summary: Add "How to Calculate Gradient" tool with i18n and page
[question] 按照默认规则，创建工具：how to calculate gradient
[try to solve] Added a new tool page and i18n entries across supported languages, registered the tool, and created a development log.
[actions]
- Updated i18n files: src/site/i18n/en.ts, zh.ts, es.ts, fr.ts, de.ts, pt.ts, id.ts, ar.ts, ja.ts, ru.ts
- Registered tool: src/site/tools.ts
- Added page: src/pages/howToCalculateGradientPage.ts