URL routing doesn't canonicalize //, rendering 404 for new tab buttons in client

[nc7s]

That's actually two problems:

• Your backend routing doesn't canonicalize //, so //games isn't canonicalized to and recognized as /games, which should be.
• The base URL constant has a slash, while added components have one too.
  • https://github.com/itchio/itch/blob/102011c51dc26a3142795c67394c365bb09cb309/src/common/constants/urls.ts#L1-L2
  • https://github.com/itchio/itch/blob/102011c51dc26a3142795c67394c365bb09cb309/src/renderer/pages/BrowserPage/newTabItems.ts#L33-L35

[3ter]

This seems like a valid improvement. Perhaps a good first issue.