Security: replace innerHTML with safe DOM rendering in auth navbar

[parakramgambhir14]

• This PR fixes a DOM-based XSS vulnerability in the authentication navbar and profile rendering system.
• Previously, user data (name and email) was injected into the DOM using unsafe HTML rendering methods. This could potentially allow script injection if malicious data is stored in localStorage.

closes #448

Fix

• Replaced unsafe HTML rendering with safe DOM APIs
• Used textContent for all user-controlled fields
• Ensured authentication UI renders without executing injected HTML

Security Impact

• Removes DOM-based XSS risk in authentication-related UI components.

[parakramgambhir14]

Hey @PatelHarsh2006 , This PR is under GSSoC'2026. Kindly add the relevant tag.