fix: anchor Start Coding/Donate buttons to sidebar bottom when Jump To nav is collapsed

The change

Resolves #1434 Problem On pages that have both the Primary nav and the Secondary ("Jump To") nav (e.g. Reference, Tutorials, Contribute, Community, contributor-doc pages), collapsing the Jump To (secondary) nav left the Start Coding and Donate buttons stuck in the middle of the side panel. They should drop to the bottom of the primary nav — the way they already do on the Homepage. Changes All changes are in src/components/Nav/styles.module.scss and scoped to the breakpoint-tablet media query (mobile is untouched). Drive side-nav heights with flexbox: Resulting behavior (tablet+) • Homepage (no Jump To): .mainlinks.noJumpTo fills the viewport; buttons sit at the bottom. ✅ • Both navs open (desktop default): primary nav stays compact; Jump To fills the remainder and scrolls. ✅ • Jump To collapsed (the bug): primary nav now fills the sidebar; buttons drop to the bottom, matching the homepage. ✅ Screenshots <img width="1413" height="882" alt="image" src="https://github.com/user-attachments/assets/b8556c90-6f11-4f66-833a-0ba7fb8a804d" /> <img width="1424" height="885" alt="image" src="https://github.com/user-attachments/assets/ace0e04e-26df-4c99-a1e2-168d7d285f65" /> Testing • ✓ npm run lint • ✓ Manual: open a Reference page at desktop width → collapse the Jump To nav → confirm Start Coding and Donate sit at the bottom of the sidebar • ✓ Manual: Reference page, Jump To open → primary nav stays compact, Jump To fills/scrolls (no regression) • ✓ Manual: Homepage → buttons at the bottom (no regression)