Frontend crashes when backend returns non-JSON or empty responses

### **_Problem_**
The frontend assumes that all API responses are valid JSON and directly calls `response.json()` without validation.

When the backend returns an empty response body or a non-JSON response (for example during authentication failures or server-side errors), the frontend crashes with the following error:

### **_Expected Behavior_**

The frontend should handle error responses gracefully and avoid crashing when the response body is empty or not valid JSON.
Instead, it should:

- Safely parse responses
- Display a user-friendly error message
- Allow the application to continue running

### **_Actual Behavior_**

The frontend crashes immediately when response.json() is called on an empty or invalid response body, making the application unusable.

### **_Impact:_**

Poor user experience

Application crashes instead of showing meaningful errors

Backend issues are masked by frontend runtime errors

New users and contributors cannot proceed when an API error occurs

### **_Steps to Reproduce:_**

Run the backend and frontend locally

Trigger an API error (e.g. auth failure or backend returning an empty response)

Observe the frontend crash with Unexpected end of JSON input

### **_Suggested Fix_**

Before calling response.json(), the frontend should:

_- Check response.ok_
_- Gracefully fall back to response.text() when JSON parsing fails_
_- Handle empty responses safely_
_- This would make the frontend more robust even when backend errors occur.__

Related Issues

This issue is exposed by backend auth failures (see issue #243), but frontend error handling should be improved independently to prevent crashes.

Start by locating the frontend API call sites that directly invoke response.json(), then reproduce the issue with an authentication failure or empty backend response. Verify the existing response handling and identify how user-facing errors are displayed. Done means invalid or empty responses no longer crash the application, errors are understandable, and normal frontend operation continues.

Written by the indexing model from the issue text.