feat(website): serve an install script that picks brew or the release binary

curl -fsSL https://re-invention.nl/soulseek-rs/install.sh | sh replaces the
brew one-liner as the front-page command: it works on Linux without Homebrew
by downloading the latest release archive for the platform, verifying its
sha256 and installing to /usr/local/bin or ~/.local/bin. Windows still gets
pointed at the msvc zip. scripts/test-install.sh covers target detection,
the brew fast path, checksum failure and real installs over curl and wget.

website/src/lib/links.ts

@@ -18,3 +18,5 @@ export const LINKS = {
 export const VERSION = __APP_VERSION__
 
 export const SITE_URL = 'https://re-invention.nl/soulseek-rs/'
+
+export const INSTALL_CMD = `curl -fsSL ${SITE_URL}install.sh | sh`