Consolidate word filtering into shared isJunkWord helper

Replace copy-pasted hasLetter/containsDigit/containsSpace/containsNonLatin
checks across 9 loaders with a single isJunkWord() call. SCOWL and CEDICT
keep their own filtering for valid reasons (SCOWL rejects hyphens/apostrophes,
CEDICT handles non-Latin Chinese characters).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/loader/omw.go

@@ -103,7 +103,7 @@ func (OMWLoader) Load(db *sql.DB, dataDir string) error {
 			continue
 		}
 
-		if word == "" || containsDigit(word) || containsSpace(word) {
+		if isJunkWord(word) {
 			continue
 		}