feat: add collapsible global sidebar

[nblog]

Summary

• add a persistent collapse/expand control to the global sidebar
• switch the collapsed sidebar to a compact icon-only layout with accessible labels and tooltips
• keep the brand mark visible while hiding the wordmark, slogan, labels, and version in compact mode
• add Chinese and English toggle labels plus regression coverage

Test plan

• npm run test:ui -- GlobalSidebar.spec.tsx --run (2 tests)
• npm run test -- i18n.test.ts --run (5 tests)
• ESLint on the three changed TypeScript/TSX files (0 errors; existing <img> warning only)
• git diff --check
• sensitive-data scans on the staged diff (0 new hits)
• Full npm run typecheck currently fails on pre-existing EnvConfig / DefaultModelSettings errors in EnvConfigDialog.tsx and SettingsPage.tsx
• Full npm run lint currently fails on existing repository-wide lint debt unrelated to this change

[CLAassistant]

All committers have signed the CLA.