Site: fit the hero's CTA column to the shell's full width

[bharathm03]

What

The hero's CTA column (OS-matched download button + subtext) got shorter after a recent copy trim, and its fixed 21rem width cap left it well short of the shell's right edge — creating a large empty block above the app window mockup, top-aligned against a two-paragraph left column.

Fix

• Grew the text column's width cap from 35rem to 45rem, so the two columns (45rem + 21rem) plus the existing gap now add up to exactly the shell's full content width. The button lands flush against the shell's right edge — the same edge the nav's own CTA sits against — with a normal gap from the text, instead of either a dead gap trailing past it or an inflated gap pushing it away.
• Self-centers the CTA column against the text column's height, so the button+subtext block sits level with the middle of the (now two-line, was three-line) paragraphs rather than pinned to the top of the row.

Verification

• bunx astro check — 0 errors, 0 warnings, 0 hints
• bun run build — 11 pages, clean
• bun run test — 68 passed