Clean up some Astro and Docs stuff

[mdo]

• Enabled prettier-plugin-astro in site/.prettierrc.json so docs-prettier-check actually checks .astro files, then ran --write across the site to catch drift it had been silently missing.
• Wrapped the adjacent <Stylesheet /> / <ScssProd.default /> in Head.astro in a JSX fragment so the newly-enabled Astro Prettier plugin parses it cleanly.
• Deleted the dead site/src/assets/snippets.js (nothing imported it) and removed its entry from eslint.config.js.
• Added a @bootstrap path alias (tsconfig + Vite resolve.alias) pointing at dist/js/bootstrap.bundle.js, and replaced every ../../../../dist/js/bootstrap.bundle.js import with it — 7 files: site/src/assets/partials/snippets.js, site/src/assets/partials/toc.js, site/src/libs/clipboard.ts, site/src/components/shortcodes/Code.astro, ButtonPlayground.astro, MenuPlacementPlayground.astro, NavbarPlacementPlayground.astro. Keeps every docs script importing from a single module instance (no duplicated component registries).
• Harmonized the "internal docs helper, don't reuse this" banner comment across application.js, search.js, partials/sidebar.js, partials/sticky.js, partials/toc.js, partials/snippets.js, and stackblitz.js.
• Dropped the unused @assets/* tsconfig path alias.
• Removed the unused layout prop from Stylesheet.astro (and the two callers in Head.astro / ExamplesLayout.astro).
• Added an explanatory comment next to the is:inline color-modes.js references in Head.astro and ExamplesLayout.astro documenting why it stays outside the Vite pipeline (needs to render-block pre-paint to avoid a flash of the wrong theme).
• Looked at collapsing DocsScripts.astro into Scripts.astro but Astro bundles <script> per component file, so inlining it under a {layout === 'docs' && ...} guard either breaks bundling or always-loads the StackBlitz SDK — kept it split and added a comment explaining why.
• Exported SidebarGroup / SidebarItem / SidebarSubItem types inferred from the Zod sidebar schema in libs/data.ts, and swapped the any casts in DocsSidebar.astro + DocsLayout.astro for the real types. Added a null guard on item.title in DocsSidebar.astro since the schema field is optional.
• Consolidated stripMarkdown / processMarkdownToHtml in libs/utils.ts onto a single remark + remark-html pipeline, dropping the direct mdast-util-from-markdown / mdast-util-to-string imports in the process.
• Removed the @oddbird/css-anchor-positioning polyfill <script> from Head.astro and uninstalled the devDep — nothing in v6-dev actually declares anchor-name (that work lives on the unmerged feature/popover-api-anchor-positioning branch), so the polyfill was a no-op on every page. Easy to add back when that branch lands.