Add Mandarin (zh) language support

Add CC-CEDICT loader and Wiktionary Chinese as data sources for
Mandarin. Also fix all Wiktionary download sections to retain
compressed archives and save checksums for integrity verification.

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

internal/loader/wiktionary.go

@@ -52,6 +52,7 @@ var wiktLangCodeMap = map[string]string{
 	"fr": "fr",
 	"pt": "pt-PT",
 	"en": "en",
+	"zh": "zh",
 }
 
 const (