Advertise the Linux desktop app on the landing page

[SawyerHood]

Human comments

What was wrong

The desktop-latest release has published a Linux x64 AppImage and a desktop-version-linux.json feed since desktop 0.42, but getbb.app only advertised "Download for macOS" everywhere and told Linux visitors to use npx. There was no Linux download redirect on the site at all.

What changed

• apps/web/src/landing/site.ts: a DesktopPlatform type and a DESKTOP_DOWNLOADS table (labels, installer extension, version feed URL, redirect path) replace the macOS-only constants. downloadHref(platform, placement) replaces downloadMacosHref.
• apps/web/src/landing/endpoints.ts: handleDownload(platform, ...) generalizes the macOS redirect. It fetches the platform's feed, picks the first asset with that platform's installer extension, and falls back to the release page. The PostHog event keeps its per-platform name (landing_download_<platform>_clicked) and download_target, so the existing macOS dashboards keep working.
• apps/web/src/routes/download.linux.tsx: new /download/linux route. /download/macos now calls the shared handler.
• apps/web/src/landing/desktop-platform.ts: detectDesktopPlatform reads client hints, navigator.platform, and the UA string. It returns linux for X11 Linux (not Android or ChromeOS), macos for Macs (not iPads reporting MacIntel), and null otherwise. useDesktopPlatform applies it after hydration with macOS as the SSR default.
• Hero, nav, and footer download links follow the detected platform. The hero shows an "Also for macOS" / "Also for Linux" link under the primary button, and the npx note now reads "Windows (via WSL), Intel Macs & remote machines". The Linux button uses a new inline LinuxIcon carrying the CC0-licensed Tux glyph from Simple Icons, since hugeicons free has no Linux icon.
• Windows and mobile visitors see the same page as before (macOS button plus npx).

Design note: this is the "detect the visitor's OS, primary button follows it, other platforms as a secondary link" pattern that VS Code, Cursor, Zed, and Warp use. Two alternatives were mocked up and rejected: side-by-side macOS and Linux buttons (three CTAs crowd the hero), and a static macOS button with an "Also available: Linux AppImage" line beneath (buries the Linux path for Linux visitors).

How you verified

• New tests: desktop-platform.test.ts covers client hints, X11 Linux, Android, ChromeOS, macOS, iPad-as-MacIntel, and Windows. endpoints.test.ts adds Linux feed resolution, a guard that a Linux request never returns a .dmg, and asserts the tracked event name, download_target, and placement. site-chrome.test.tsx asserts the SSR nav href stays /download/macos?placement=nav.
• pnpm exec turbo run typecheck test lint --filter=@bb/web passes (23 files, 124 tests).
• Manual, against vite dev: curl -I /download/linux returns a 302 to bb-0.42.1-x86_64.AppImage; /download/macos still returns the 0.42.1 dmg. In headless Chrome on Linux the nav and hero show "Download for Linux" with hrefs to /download/linux; with a macOS UA and MacIntel platform emulated they show "Download for macOS". Checked the 390px dark-mode layout as well.

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