Add Multi-language Support (i18n) for Backend and Frontend

[google-labs-jules]

This pull request implements comprehensive internationalization (i18n) support for the POS application.

Backend Changes:

• Translation Management: Enhanced back/app/messages.py to store and retrieve localized strings for 6 languages: English, Spanish, Catalan, German, Chinese, and Hindi.
• Language Detection: Added back/app/language_service.py to parse the Accept-Language header or lang query parameter, normalizing language codes (e.g., zh -> zh-CN).
• Route Updates: Refactored back/app/main.py and back/app/inventory_routes.py to inject the detected language and use get_message() for generating HTTPException details, ensuring API errors are returned in the user's preferred language.

Frontend Changes:

• Translation Files: Updated JSON files in front/public/i18n/ to include missing translation keys for:
  • Inventory Categories (e.g., INVENTORY.CATEGORIES.INGREDIENTS)
  • Units of Measure (e.g., INVENTORY.UNITS.PIECE)
  • Generic Catalog labels (e.g., CATALOG.ALL_CATEGORIES)
  • Common terms like NONE.
• Component Updates:
  • ProductsComponent: Now uses translate pipe for category filters and translateService for API error messages.
  • InventoryItemsComponent: Replaced hardcoded string formatting for categories and units with dynamic translations using the new JSON structure.

This ensures a consistent multi-language experience across both the UI and API responses.

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PR created automatically by Jules for task 15334820750941838496 started by @wiatakillo

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