Make one image URL per source on the generator page for host case and an empty path (closes #293)

The generator page put the source host and path into the encrypted URL
as entered, so CDN.Example.com and cdn.example.com, or a source URL with
no path and with the path /, made two cache keys and two upstream
requests for one image. It now parses the source URL with the host
lower-cased and an empty path written as /, as /v1/image/ reads a
source URL, before the check that pixa fetches the URL as entered. That
is a small function of its own because HandleGenerateURL is at the
linter's length limit. The README says what the token keeps.

Model: opus-5-5

TODO.md

@@ -30,6 +30,11 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-16 the generator page lower-cases the source host and writes an empty
+  source path as `/` before it makes the encrypted URL, as `/v1/image/` does
+  (closes #293). A host written in upper and in lower case, or a source URL with
+  no path and with the path `/`, made two cache keys and two upstream requests
+  for one image.
 - 2026-10-16 the source URL a request fetches is built with `net/url` from the
   source host, path and query, not joined as text (closes #288). The source
   path, decoded, is fetched percent-encoded again, so `100%25.png` fetches