refactor(i18n): consistent empty input handling and add doc comment

- Article("") now returns "" for consistency with other grammar
  functions (PastTense, Gerund, PluralForm, Label all return "")
- Add doc comment to getMessage() for consistency with other
  internal helpers

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

pkg/i18n/grammar.go

@@ -356,7 +356,7 @@ func applyRegularPlural(noun string) string {
 //	Article("hour")     // "an" (silent h)
 func Article(word string) string {
 	if word == "" {
-		return "a"
+		return ""
 	}
 
 	lower := strings.ToLower(strings.TrimSpace(word))

pkg/i18n/grammar_test.go

@@ -243,7 +243,7 @@ func TestArticle(t *testing.T) {
 		{"heir", "an"},
 
 		// Edge cases
-		{"", "a"},
+		{"", ""},
 		{" error ", "an"},
 	}
 

pkg/i18n/service.go

@@ -491,6 +491,8 @@ func (s *Service) Raw(messageID string, args ...any) string {
 	return text
 }
 
+// getMessage retrieves a message by language and key.
+// Returns the message and true if found, or empty Message and false if not.
 func (s *Service) getMessage(lang, key string) (Message, bool) {
 	msgs, ok := s.messages[lang]
 	if !ok {