Split the install page into Linux / macOS / Windows tabs so each visitor
sees only the commands that apply to them. A small InstallOsDetect component
seeds Starlight's synced-tabs localStorage key with the detected OS before
the tabs render, so the matching tab is pre-selected with no flash — and
only when the visitor hasn't already picked a tab, so a manual choice wins.
- installation.md -> .mdx (needs the Tabs/TabItem components)
- detection covers navigator.userAgentData and the legacy platform/UA
fallbacks; unknown platforms fall through to the default tab
- piggybacks on Starlight's own restore mechanism rather than re-implementing
tab switching
