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/res/values/strings.xml

@@ -74,7 +74,7 @@
     <string name="dictionary_source_duplicate_prefix">This dictionary already exists</string>
     <string name="dictionary_source_url_empty">URL cannot be empty</string>
     <string name="dictionary_source_invalid_protocol">Must start with http:// or https://</string>
-    <string name="dictionary_source_invalid_extension">Must end with .gz, .dsl, or .dz</string>
+    <string name="dictionary_source_invalid_extension">Must end with .gz, .dsl, or .dsl.dz</string>
     <string name="dictionary_source_too_long">URL is too long (max 2048 characters)</string>
     <string name="dictionary_source_invalid_url">Invalid URL format</string>
     <string name="dictionary_source_suspicious_characters">URL contains invalid characters</string>