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

The source path, decoded, is fetched percent-encoded again by net/url,
so 100%25.png fetches /100%25.png, and my%20cat.png and my%2520cat.png
fetch different paths. The source query stays as the source URL writes
it; the characters a browser percent-encodes in a query, such as a
space, are percent-encoded when the image URL is read, so the query is
signed and cached as it is sent. The generator page answers 400 for a
source URL its image URL would fetch written another way, such as with
%2F in the path, and shows the URL pixa would fetch. README.md says how
the source path and query are fetched.

Model: opus-5-5

internal/imgcache/urlparser.go

@@ -51,6 +51,12 @@ var sizeFormatRegex = regexp.MustCompile(
 // percentEscapeRegex matches one percent-escape, such as "%2e"
 var percentEscapeRegex = regexp.MustCompile(`%[0-9A-Fa-f]{2}`)
 
+// queryEscapeRegex matches a percent-escape, or one character that a browser
+// percent-encodes in a URL's query: one outside the printable ASCII characters
+// "!" to "~", such as a space, a control character or a character outside
+// ASCII, or one of `"#<>`. A "%" that does not begin a percent-escape matches too.
+var queryEscapeRegex = regexp.MustCompile(`%[0-9A-Fa-f]{2}|[^!-~]|["#<>%]`)
+
 // hostRegex matches the forms the host of an image URL may take: a host name
 // or an IPv4 address (letters, digits, "-", "_" and "."), or an IPv6 address
 // in brackets (hex digits, ":" and "."), either one optionally followed by ":"
@@ -179,9 +185,12 @@ func parseImageComponents(remainder string) (*ParsedURL, error) {
 			`%w: the source path or query holds a "#"`, ErrInvalidPath)
 	}
 
-	// Extract query string if present in path
+	// Extract query string if present in path. It is the source URL's query
+	// as that URL writes it. A character a URL's query cannot hold as it is,
+	// such as a space written %20 in the image URL, is percent-encoded here,
+	// so the query is signed and cached as it is sent upstream
 	if qIndex := strings.Index(path, "?"); qIndex != -1 {
-		query = path[qIndex+1:]
+		query = escapeQuery(path[qIndex+1:])
 		path = path[:qIndex]
 	}
 
@@ -270,6 +279,20 @@ func decodePercentEscapes(s string) string {
 	})
 }
 
+// escapeQuery returns a source query as a URL's query holds it: each
+// character that queryEscapeRegex names, such as a space, percent-encoded,
+// and the rest, the query's percent-escapes included, kept as they are. The
+// result is unchanged by a second escapeQuery.
+func escapeQuery(query string) string {
+	return queryEscapeRegex.ReplaceAllStringFunc(query, func(match string) string {
+		if percentEscapeRegex.MatchString(match) {
+			return match
+		}
+
+		return url.PathEscape(match)
+	})
+}
+
 // parseSizeFormat parses strings like "800x600.webp", "orig.png" or "800x600"
 func parseSizeFormat(s string) (Size, ImageFormat, error) {
 	matches := sizeFormatRegex.FindStringSubmatch(s)