Make mobile menu fill full viewport height

[mre]

Summary

The mobile navigation menu now always uses the full viewport height when open, instead of relying on large fixed padding per item.

Problem

• Each menu item had padding: 2.4em top and bottom. With the current 7 items this made the open menu taller than a typical phone screen, causing overflow.

Change (sass/_base.scss, mobile @media (max-width: 950px) block)

• When open, the menu spans 100dvh (#menu-toggle:checked ~ .menu), using dvh so it accounts for mobile browser chrome.
• Items use flex: 1 1 0 so they grow to share the available height evenly. This means the menu always fills the screen exactly with no leftover gaps, regardless of how many items there are.
• Links/CTA now center their content via flexbox and fill their row (height: 100%) instead of using fixed 2.4em padding.

Result

• Fits any phone screen without overflow.
• No empty gap at the top or bottom.
• Scales automatically if menu items are added or removed.