Fix: ID card generation TypeError due to API response change

[sentry]

This PR addresses issue SH-STAFF-APP-E, a TypeError: Cannot read properties of undefined (reading 'operation') occurring during ID card generation.

Root Cause:
The generate method in candidate.id.card.service.ts was calling this._authhttp.generateCards. This generateCards method was designed to handle blob responses (e.g., for file downloads) and explicitly returned undefined after processing the blob. However, the backend API for ID card generation has changed to return a JSON object ({ operation: 'success', cir_uuid: '...' }) for asynchronous processing, not a blob. Consequently, the caller in candidate-list.page.ts received undefined and crashed when attempting to access response.operation.

Solution:
Updated the generate method in candidate.id.card.service.ts to use this._authhttp.post(url, { candidates }) instead of this._authhttp.generateCards. This ensures that the JSON response from the API is correctly returned to the calling component, allowing the response.operation property to be accessed as expected. The filename variable, which was only relevant for the blob download functionality, has also been removed as it is no longer needed.

Fixes SH-STAFF-APP-E
Fixes TECH-2058

[linear-code]

TECH-2058

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