Fix: Add project button missing minimum touch target height on mobile

## TL;DR
The Add project button at the bottom of the sidebar renders below 44px height on mobile, making it difficult to tap.

## Current State
The button has no minimum height. On mobile it renders at natural text height (~28px).

## Desired State
The button should include min-h-[44px] and flex items-center to meet the 44px touch target requirement from the UI spec.