Fix login modal positioning on mobile devices

[Copilot]

The login modal was positioned at the bottom of the screen on mobile devices instead of being properly centered, creating a poor user experience. This was particularly problematic on smaller viewports where the modal appeared to be "stuck" at the bottom.

Problem

On mobile devices, the HeroUI Modal component was using default positioning which resulted in bottom placement. This made the login form difficult to access and provided inconsistent UX compared to desktop.

Solution

Added the placement="center" prop to the Modal component in login-modal.tsx. This leverages HeroUI's built-in positioning system which provides:

• "center" - Centers the modal in the viewport (our fix)
• "auto" - Default behavior (was causing the issue)
• "top", "top-center", "bottom", "bottom-center" - Other positioning options

// Before
<Modal isOpen={isOpen} onClose={onClose}>

// After  
<Modal isOpen={isOpen} onClose={onClose} placement="center">

Testing

• ✅ Verified on multiple mobile viewport sizes (320x568, 375x667)
• ✅ All 457 unit tests passing
• ✅ Build and linting successful
• ✅ Desktop functionality preserved
• ✅ Login flow works correctly on both mobile and desktop

Screenshots

Before (❌ Modal stuck at bottom):

After (✅ Modal properly centered):

The fix also verified that other reported mobile issues (tab header wrapping and basic info form scrolling) were already working correctly without additional changes needed.

Fixes #123.

Warning

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