Replace loose equality with strict equality in SignInControl

[Copilot]

Addresses code review feedback from #1389 to use strict equality operators for type-safe comparisons.

Changes

• Replaced 7 instances of == with === in SignInControl.tsx:
  • Enum comparisons in renderSigninEmailButton() (3 instances)
  • Conditional rendering logic for form fields (3 instances)
  • Email input disabled prop (1 instance)

All changes involve comparisons against the EmailSigninStep enum:

// Before
if (emailSignInStep == EmailSigninStep.EnterEmail) { ... }

// After  
if (emailSignInStep === EmailSigninStep.EnterEmail) { ... }

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