New chat landing centering

The new-chat landing stack now follows Claude's measured positioning: pt-[10vh] md:pt-[20vh]. Horizontal centering and the 672px composer width are unchanged.

Desktop local 209.455px / 20vh
Desktop expected 209.4px
Mobile local 76.727px / 10vh
Mobile expected 76.7px
Desktop after Authenticated local app, desktop viewport.
Desktop new-chat landing after vertical alignment
Mobile after Authenticated local app, narrow viewport.
Mobile new-chat landing after vertical alignment