赵月辉 / fastgpt-migrated

* fix(document): improve mobile sidebar style and UX

- Add icons to 6 navigation tabs (BookOpen, Code, Lightbulb, CircleHelp, Scale, History)
- Reduce mobile sidebar padding and separator spacing
- Fix icon container size in RootToggle dropdown (36px → 20px)
- Keep sidebar open when switching between top-level tabs
- Remove top gradient mask on mobile sidebar viewport
- Fix long URL overflow in article content on mobile
- Fix TOC popover trigger text overflow on mobile

* docs: 中国大陆 → 中国大陆版

* docs: 海外版→国际版, 国内版→中国大陆版

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Co-authored-by: archer <archer@archerdeMac-mini.local>