fix(settings): accept dictionary source URLs over https only

The platform blocks cleartext traffic by default, so an http:// source
passed validation but could never download and failed with a misleading
network error. The validator now rejects http:// upfront.

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

@@ -106,7 +106,7 @@ data class DictionarySource(
             return when {
                 trimmed.isBlank() -> ValidationResult.Error(ValidationError.EMPTY)
                 trimmed.length > 2048 -> ValidationResult.Error(ValidationError.TOO_LONG)
-                !trimmed.startsWith("http://") && !trimmed.startsWith("https://") ->
+                !trimmed.startsWith("https://") ->
                     ValidationResult.Error(ValidationError.INVALID_PROTOCOL)
                 !lowerTemplate.endsWith(".gz") &&
                     !lowerTemplate.endsWith(".dsl") &&