I18n fr/product

[MaximeNzo]

Added French (fr) localization for the Product section and i18n infrastructure

Summary

• Add full French translation of the Product section (Features, Currency Conversion, Workstreams) under /fr/product/
• Configured VuePress with locale-specific navigation, sidebar, and language selector for English (/) and French (/fr/)
• Added a locale-aware 404 page that displays French messages and links when on /fr/ paths
• Implemented automatic language detection in enhanceApp.js that redirects French-speaking browsers to the /fr/ equivalent of the
  current page on first visit
• Translated Currency Conversion diagrams (PlantUML sources and SVG exports) to reflect French terminology
• Fixed image paths in French files to account for the extra directory depth (docs/fr/product/... vs docs/product/...)

Test plan

• Verify the language dropdown appears and switching between English/Français works
• Navigate through all French Product sidebar pages and confirm content renders with images
• Click TOC links on Feature documents and confirm scrolling works correctly
• Visit /fr/community/ and confirm the French 404 page is shown
• Open the site in a French-language browser and confirm auto-redirect to /fr/
• Confirm English pages remain unaffected

[kayiganwa04]

@MaximeNzo great job here added my comments, some of the messy codes might even be comming from my previous PR.