Replace the placeholder Steam store URL on the download page

[charneykaye]

Replace the placeholder Steam store URL on the download page.

website/content/download.md currently links the "Buy on Steam — $9.99" card and the store reference to https://store.steampowered.com/ as a placeholder, with an HTML comment marking the swap:

<!-- Replace the Steam href above with the live store URL once the App ID is
     registered: https://store.steampowered.com/app/<STEAM_APP_ID>/ -->

• Once the Steam app is registered (see the Steam registration issue), replace the placeholder href with the live https://store.steampowered.com/app/<STEAM_APP_ID>/ URL.
• Do the same anywhere else the store is linked (homepage CTA in website/layouts/index.html points at /download/, so it inherits the fix — verify).

Small, but it's the actual buy button, so it should land with launch.