Carry request path through 404 page links for smoother OAuth redirects

[oosawy]

As mentioned in the google-oauth example, using plain localhost as the OAuth redirect URI means you lose the portless benefits for the callback flow. But the callback does land on portless's 404 page, which already lists active apps — you can manually click through to the right one. The only thing missing is that the path and query string get dropped along the way.

Changes

• Append req.url to the active-app links on the 404 page so the full path and query string carry over
• For OAuth callbacks this means one click instead of manually reconstructing the URL
• Root-path requests (/) produce clean links with no trailing slash

Test plan

• pnpm test — 3 new test cases (path preserved, path + query preserved, root path omitted)
• Existing proxy tests pass without changes

If desired, we could also surface an extra hint on the 404 page when the request has a non-root path, but kept it minimal for now.

[vercel]

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