fix(#981): prevent multiple duplicate success toasts on guest login

[sidhanta03]

Description

Resolves #981.

Rapidly clicking the "Sign in as Guest" button previously caused multiple identical API requests
and overlapping UI success toasts.

Changes made:

• Implemented a synchronous JavaScript lock using useRef to securely block rapid double/triple
  clicks — mathematically impossible for 2 clicks to pass the guard since refs mutate synchronously
• Added setIsSubmit(true) for visual UI feedback while request is pending
• Added a setTimeout cooldown inside the finally block to prevent aggressive spamming on
  fast-resolving errors
• Used try/finally pattern to ensure lock always releases on both success and failure

What type of PR is this?

• 🐛 Bug Fix

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in
  hard-to-understand areas
• I have added tests
• Existing tests pass locally

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud