Fix 920

[fahatadam]

PR Title:
fix: type and test backend i18n middleware and localized error responses

Description:

What was done:

• Removed // @ts-nocheck suppressions from backend/src/index.ts and backend/src/routes/index.ts to un-suppress the i18n routing path and enforce type checking.
• Updated backend/src/middleware/i18n.ts to robustly parse req.headers['accept-language'] arrays, explicitly enforcing safe type handling for the locale negotiation.
• Created and exported a LocalizedError class in backend/src/middleware/errorHandler.ts to create a strictly typed contract for translating backend errors.
• Updated the global errorHandler to intercept LocalizedError instances, automatically fetching the localized message with req.t(err.key), while safely falling back to a generic 500 error for standard native errors.
• Added comprehensive Jest tests for i18n header fallback logic (i18n.test.ts) and global error handler fallback safety (errorHandler.test.ts).

Why it was done:
To establish a typed and testable contract for locale selection and safe translated errors without suppressing type safety on the backend. This guarantees deterministic locale negotiation and prevents sensitive error information from leaking out during runtime exceptions.

How it was verified:

• Verified by adding targeted unit tests that cover header negotiations, unsupported locales, array edge-cases, and error localization fallback flows.
• Verified TypeScript compilation constraints are preserved successfully after the removal of the @ts-nocheck directives.

Closes #920

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