feat: add English frontend translation coverage

[RomuloGatto]

Summary

• Keep Simplified Chinese as the default language; English is only restored when explicitly selected via jellyfish_language=en-US.
• Move main layout navigation and breadcrumb labels into zh-CN/en-US translation resources.
• Add an English translation coverage layer for existing hardcoded frontend UI text so English mode renders current screens in English while the app migrates hardcoded labels to first-class i18n keys.
• Expand the English UI dictionary to cover extracted Chinese frontend UI literals, JSX text fragments, template literal fragments, and common runtime labels.
• Fix the interim DOM translator to prefer longer source phrases first so short keys cannot partially corrupt longer labels.
• Fix spaced Ant Design two-character button labels such as 取 消 by matching compact Chinese source phrases before translating.
• Fix Settings page namespace usage so it resolves real localized strings instead of displaying raw i18n keys.

Test Plan

• Static scan: no remaining uncovered Chinese JSX/template literal fragments outside locale/generated files.
• cd front && pnpm exec tsc --noEmit
• cd front && pnpm run build
• Browser verification on local Vite app:
  • default/no stored language renders Simplified Chinese on /projects
  • explicit English selection renders /projects, /agents, /models, /assets, /prompts, /files, and /settings with no Chinese text detected in document.body.innerText
  • verified the New Project modal renders the footer buttons as Cancel and Create and enter, with no Chinese text detected
  • verified /settings no longer displays raw settings.* keys.