Fix header height regression: negative margin on sign-out button
The min-h-[44px] on the sign-out button was stretching the header from
53px to 69px on all viewports. Adding -my-2.5 lets the 44px hit area
extend into the header padding without growing the header itself.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>