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/res/values/strings.xml

@@ -74,7 +74,7 @@
     <string name="dictionary_source_duplicate_in_form">Duplicate URL in the form</string>
     <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_protocol">Must start with https://</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>