test(frontend): validate SignIn return-route handling

[utksh1]

Summary

Add routing tests that accept safe internal return paths and reject external or malformed redirect targets.

Scope

• Suggested file: frontend/src/pages/SignIn.tsx
• Add or extend a focused frontend unit test.
• Do not bundle unrelated visual or production changes.

Acceptance criteria

• The behavior is covered with a deterministic test.
• The user-visible or accessibility outcome is asserted.
• Existing tests remain stable.
• The focused frontend test passes.

Verification

Run the affected Vitest file from frontend/.

[Ritikyadav2004]

/assign

[Sanskriti-Agarwala]

Hi! I'd like to work on this issue.
Could you please assign it to me?

[Ritikyadav2004]

Hi utksh1,

I have submitted PR #2352 for issue #2178 covering the SignIn return-route handling tests. Could you please take a look and review/merge it when you have a moment? Let me know if any changes are needed!