Add Get started page and header CTA
Adds a /get-started landing page (CLI, Desktop, CI, AI agent sections) and a brand-blue "Get started with Radicle" button in the global header that links to it. Reuses the existing Button secondary variant; swaps its base/hover blues so the brighter shade is the resting state.
Get started: bump side padding to 4rem
Get started: fix mobile overflow and tweak header CTA
- Show CTA on mobile, but hide it on the Get started page itself
- Shrink heading to xl on mobile
- Add min-width: 0 / width: 100% chain so the long CLI install command no longer expands the grid track and overflows the page
Show slim Header on repo views on mobile