Fix root grid on mobile and line length

src/routes/+layout.svelte lays out header, sidebar, content and footer in a grid-template-columns: 1fr 4fr grid with margin-inline-end: 2em and no media query.

Current State

• 390px viewport: sidebar column takes 59px, right margin 34px, content gets 297px
• The left column is reserved even where there is no Nav (home, listings, glossary), so every page starts 20% from the left edge
• 1280px viewport: article text is 993px wide at 19px, roughly 110 characters per line; .ids__wrapper (70%) and .ids__text-width from the design system are unused

Tasks

• Collapse to a single column below 768px; move the Nav inline or hide it
• Only allocate the sidebar column when the page registers sections, or overlay the Nav instead of reserving grid space
• Cap the article measure at around 65–75 characters (max-width in ch on article section) and let media stay wide