[datlechin]

Second of three PRs for the Linux app. /download rebuilt as platform tabs, after tailscale.com/download. Linux shows only once platforms.json says released (PR 3 flips it), so after this merges the live page shows Mac and iPhone & iPad.

• Downloads only: no feature table, no engine counts, no screenshots. What runs where stays on the feature pages and /databases.
• Header: app icon and H1, centred, no lead.
• Switch: Mac | iPhone & iPad | Linux (Beta), one panel at a time. The visitor's platform is preselected.
• No swap on load: the head script's ios / linux root class shows the right panel before paint, and React keeps the same choice. CLS 0 for Mac, Linux and iPhone visitors.
• #mac, #ios, #linux open their panel; a choice goes into the URL hash. #updates (linked from Security) still lands on the switch.
• Mac panel: Download for Mac · version, date, size · requirement, architectures · updates line · Homebrew with copy · Verify your download (signing note, shasum, SHA-256).
• iPhone & iPad panel: App Store badge · version, free, requirement · updates line · /ios link.
• Linux panel: version · beta note with the issues link · Ubuntu | Fedora | Flatpak | Files tabs, one copyable block each, requirement and update line; Files lists packages by format and architecture, SHA256SUMS, Verify a file.
• One row under the switch: Release notes (Mac, Linux, Changelog) · Older versions (all releases, build from source) · "Is there a version for Windows?".
• Each fact appears once. Gone: the cards, the Updates section, the older-versions cell per platform, the latest release post, the FAQ block.
• SegmentedControl gets size="lg", and its thumb only slides after a click (a value restored on load lands in place; the Linux tab restore used to slide).
• Page height at 1440: 3,048 px before this rewrite, ~1,670 px now including the footer.
• JSON-LD gains #linux-app when Linux is released. download_click gains platform linux. The install-tab choice is stored as tablepro:linux-install; all 12 privacy policies list it.
• 12 locales. pest --exclude-group=ssr 1038 passed, --group=ssr 53 passed, test:js 229 passed, typecheck clean.

Before (production)	After, as it goes live	After, Linux released

iPhone visitor	Linux visitor, phone	Dark	Vietnamese