Refactor homepage CSS: remove :has(), scope hero, and lock sidebar width

[ttuff]

Motivation

• Remove fragile :has() selectors and scope homepage-only styles to a controlled structure so rules cannot leak to other pages.
• Keep the existing, working hero/header overlap behavior while simplifying selectors and preventing the hero from being positioned beneath the sidebar.
• Stabilize the sidebar width so it cannot expand and break layout on large screens.

Description

• Replaced all body:has(...) homepage selectors in docs/styles/extra.css with structure-rooted selectors using .oasis-layout and .oasis-main (content padding, H1 spacing, hero spacing, and kicker styling now scoped to .oasis-layout).
• Kept the hero flush-to-header rule exactly as requested under .oasis-layout .oasis-main .oasis-hero using margin-top: calc(var(--md-header-height, 3rem) * -1) and padding-top: var(--md-header-height, 3rem).
• Updated the sidebar width token to --oasis-sidebar-w: 300px and hardened desktop sidebar sizing with overflow: hidden and added .oasis-sidebar * { max-width: 100%; } to prevent children from forcing expansion.
• Ensured the hero remains confined to .oasis-main and did not reintroduce any 100vw or negative viewport-offset tricks.

Testing

• Ran rg ':has\(' docs/styles/extra.css and confirmed there are no :has() selectors remaining in the file.
• Ran mkdocs build successfully (build completes; existing documentation link warnings are pre-existing and unrelated).
• Launched a local server and captured a visual sanity-check screenshot (mkdocs serve + Playwright screenshot) to verify the hero touches the header, is not under the sidebar, and the sidebar width is stable; visual check succeeded.

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