fix: prevent janky login screen flash after successful authentication

[ritoban23]

📄 Summary

This PR fixes a janky user experience on the Sign In screen where, after submitting credentials, the login screen briefly flashes again before redirecting to the dashboard. The change ensures a smooth transition from loading to dashboard after successful authentication.

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✅ Changes

• Bug fix: Prevents login screen from flashing after successful sign-in by showing a loading spinner when the user is already authenticated.
• UI: Improves login flow for a seamless user experience.

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🏷️ Required: Add Relevant Labels

⚠️ Manually add appropriate labels in the PR sidebar
Please select one or more labels (as applicable):

• frontend
• bug
• ui
• enhancement

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👥 Reviewers

Tag the relevant teams for review:

• frontend

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🧪 How to Test

1. Start the frontend dev server.
2. Go to the login screen and sign in with valid credentials.
3. Observe the flow: after clicking "Sign In," you should see a loading spinner and then be redirected directly to the dashboard, without the login screen flashing again.
4. Try reloading /login while already authenticated; you should see a spinner and be redirected to the dashboard.

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🔍 Related Issues

Closes #9417

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📸 Screenshots / Screen Recording (if applicable / mandatory for UI related changes)

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📋 Checklist

• Dev Review
• Test cases added (Unit/ Integration / E2E)
• Manually tested the changes

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👀 Notes for Reviewers

• The fix is minimal and only affects the login flow.
• No breaking changes to other authentication or routing logic.

[welcome]

Welcome to the SigNoz community! Thank you for your first pull request and making this project better. 🤗

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