fix: align organisation dropdown on responsive viewports

[nielskaspers]

Summary

Fix the organisation selector dropdown overflowing to the left on responsive/small screen viewports, which hid organisation names.

Issue

Fixes #6607

Changes

• Changed left: -50px to left: 0 for .nav-modal at the md breakpoint in _project-nav.scss. At this breakpoint, the .inline-modal is already position: fixed with left: 0; right: 0 (set at the lg breakpoint), so the -50px offset was shifting the fullscreen modal off-screen to the left.

Testing

• Resize browser window to a small/responsive viewport (below md breakpoint)
• Click the organisation selector dropdown in the top navbar
• Verify the dropdown stays within the viewport and all org names are visible

[claude]

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