Document Sentry DSN environment variable

[Akshith-cdr]

Description

Fixes #3436

Documented the optional REACT_APP_SENTRY_DSN environment variable across the sample env file, README, and environment setup guide.

Clarified that the Sentry DSN is used for browser error reporting only in production builds and can be left blank during local development.

Type of change

• 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