fix(cli): list available languages on --lang mismatch; fix telemetry truncation

Real-world telemetry showed --lang "English (United States)" failing
for several consumer products that only list "English International"
-- the exact example used in --help. The mismatch error now lists the
languages actually available, and the help text notes names vary by
product.

Also fixes truncateError to keep both head and tail of long error
strings (previously kept only the head, which for wrapped errors with
a long URL cut off the actual root cause at the end).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cli/main.go

@@ -91,16 +91,23 @@ type cliTelemetryPayload struct {
 }
 
 // truncateError caps an error string for telemetry (avoids leaking huge bodies,
-// keeps Redis field values small).
+// keeps Redis field values small). Keeps both head and tail: the head usually
+// names what failed (e.g. "language X not available for Y"), while wrapped
+// errors put the root cause last (e.g. "fetching X: <root cause>") — a long
+// middle section (URLs, option lists) is the least useful part to keep.
 func truncateError(err error) string {
 	if err == nil {
 		return ""
 	}
 	s := err.Error()
-	if len(s) > 150 {
-		s = s[:150]
+	const max = 150
+	if len(s) <= max {
+		return s
 	}
-	return s
+	const headLen = 60
+	const sep = " ... "
+	tailLen := max - headLen - len(sep)
+	return s[:headLen] + sep + s[len(s)-tailLen:]
 }
 
 // sendTelemetry posts a single telemetry event. Fire-and-forget: all errors
@@ -161,6 +168,8 @@ Usage:
   msdl [search terms]                           interactive: filter + pick product, pick language
   msdl --id 3262                                skip product picker
   msdl --id 3262 --lang "English (United States)"  no prompts, print URL directly
+                                                 (language names vary by product — omit --lang
+                                                 or check the error's "available" list to see them)
   msdl --eval [slug]                            evaluation ISOs (server-2025, win11-ent, ...)
   msdl --list                                   list all products and exit
 
@@ -310,7 +319,11 @@ func runConsumer(productID, query, langName string, noContribute bool) error {
 			}
 		}
 		if lang.ID == "" {
-			return fmt.Errorf("language %q not available for %s", langName, product.Name)
+			names := make([]string, len(langs))
+			for i, l := range langs {
+				names[i] = l.Language
+			}
+			return fmt.Errorf("language %q not available for %s — available: %s", langName, product.Name, strings.Join(names, ", "))
 		}
 	} else {
 		lang, err = pickLanguage(langs)