fix(layout): pin the footer to the viewport bottom on short pages

[olegshulyakov]

Summary

• Makes body a min-height: 100vh flex column so .primer-layout grows to fill leftover space, landing the footer on the window's bottom edge on short pages instead of right under the content. Previously this only happened by accident, via .primer-sidebar's fixed height — which stopped applying once the sidebar hides below 1011.98px, leaving a dead gap under the footer on mobile/tablet.
• Stretches .primer-sidebar to its actual grid row (capped with max-height) instead of a flat 100vh, so on desktop it no longer overshoots the footer by its own height on short pages (which pushed the footer below the fold on tall windows), while still scrolling a long nav list within its own box on long pages.

Closes #92

Test plan

• Short page at mobile width (375px) — footer flush at bottom, no dead gap
• Short page at tablet width (768px) — footer flush at bottom, no dead gap
• Short page at desktop width with a tall window (1440x1400, the reported repro) — footer visible at bottom, no overflow past viewport
• Long page at desktop width — sidebar still caps at viewport height with its own internal scroll, footer lands at true end of page