Fetch the source path and query URL-encoded, not joined as text (closes #288)

The source URL was built by joining the decoded path and query as
text, so a space in the query reached the upstream raw and failed with
502, 100%25.png made a URL Go could not parse and answered 401, and
%20 and %2520 fetched the same path under different cache keys. The
source URL is now built with net/url, so each decoded path and query is
one upstream request matching its signature and cache key; the query
encodes the characters a browser encodes, ' included. The generator page
compares what pixa would fetch with the URL as entered, and answers 400
with the URL to use when they differ, or for a user name and password
or a .. segment.

Model: opus-5-5

internal/imgcache/imgcache.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"errors"
 	"io"
+	"net/url"
 	"time"
 )
 
@@ -86,9 +87,11 @@ func ValidateImageRequest(req *ImageRequest) error {
 type ImageRequest struct {
 	// SourceHost is the origin host (e.g., "cdn.example.com")
 	SourceHost string
-	// SourcePath is the path on the origin (e.g., "/photos/cat.jpg")
+	// SourcePath is the path on the origin with its percent-escapes decoded,
+	// as url.URL's Path holds it (e.g., "/photos/my cat.jpg")
 	SourcePath string
-	// SourceQuery is the optional query string for the origin URL
+	// SourceQuery is the optional query string for the origin URL as the URL
+	// writes it, percent-escapes included, as url.URL's RawQuery holds it
 	SourceQuery string
 	// Size is the requested output dimensions
 	Size Size
@@ -107,20 +110,25 @@ type ImageRequest struct {
 	AllowHTTP bool
 }
 
-// SourceURL returns the full upstream URL to fetch.
-// Uses http:// scheme when AllowHTTP is true, otherwise https://.
+// SourceURL returns the full upstream URL to fetch: the source path
+// percent-encoded again, as net/url writes a path, so each source path is one
+// upstream path, and the source query as it is, with the characters
+// escapeQuery encodes percent-encoded. Uses http:// scheme when AllowHTTP is
+// true, otherwise https://.
 func (r *ImageRequest) SourceURL() string {
 	scheme := "https"
 	if r.AllowHTTP {
 		scheme = "http"
 	}
 
-	url := scheme + "://" + r.SourceHost + r.SourcePath
-	if r.SourceQuery != "" {
-		url += "?" + r.SourceQuery
+	sourceURL := url.URL{
+		Scheme:   scheme,
+		Host:     r.SourceHost,
+		Path:     r.SourcePath,
+		RawQuery: escapeQuery(r.SourceQuery),
 	}
 
-	return url
+	return sourceURL.String()
 }
 
 // ImageResponse represents a processed image ready to serve