Align SSE API environment variable

[Akshith-cdr]

Description

Fixes #3433

Aligned the SSE EventSource base URL with the documented environment variable name by using REACT_APP_API_URL consistently. Updated the environment docs to clarify that REACT_APP_API_URL is used for both normal client API calls and SSE stream endpoints.

Added a regression assertion to the SSE multiplexer test to verify EventSource URLs are built from REACT_APP_API_URL.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• This change requires a documentation update

Screenshots / Video (if applicable)

N/A

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have run local unit tests using npm test and verified they pass
• I have run npm run lint and resolved any new errors or warnings
• I have verified responsiveness and visual alignment on desktop and mobile viewports