fix: reject bare .dz source URLs and make extension validation case-insensitive

A bare .dz file (DictZip without the .dsl prefix) is treated by the
pipeline as an orphaned file and deleted right after the download, so
a .dz source URL passed validation but silently produced no
dictionary. Accept only DictZip-compressed DSL (.dsl.dz; .gz already
covers .dsl.gz) and compare extensions case-insensitively.

app/src/main/java/com/example/research/core/domain/model/DictionarySource.kt

@@ -102,12 +102,15 @@ data class DictionarySource(
 
         fun validateTemplate(template: String): ValidationResult {
             val trimmed = template.trim()
+            val lowerTemplate = trimmed.lowercase()
             return when {
                 trimmed.isBlank() -> ValidationResult.Error(ValidationError.EMPTY)
                 trimmed.length > 2048 -> ValidationResult.Error(ValidationError.TOO_LONG)
                 !trimmed.startsWith("http://") && !trimmed.startsWith("https://") ->
                     ValidationResult.Error(ValidationError.INVALID_PROTOCOL)
-                !trimmed.endsWith(".gz") && !trimmed.endsWith(".dsl") && !trimmed.endsWith(".dz") ->
+                !lowerTemplate.endsWith(".gz") &&
+                    !lowerTemplate.endsWith(".dsl") &&
+                    !lowerTemplate.endsWith(".dsl.dz") ->
                     ValidationResult.Error(ValidationError.INVALID_EXTENSION)
                 !isValidUrlStructure(trimmed) ->
                     ValidationResult.Error(ValidationError.INVALID_URL_STRUCTURE)