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/hunspell.go

@@ -57,7 +57,7 @@ func loadHunspell(db *sql.DB, path, lang string) error {
 		word := strings.SplitN(line, "/", 2)[0]
 		word = strings.ToLower(word)
 
-		if !hasLetter(word) || containsDigit(word) || containsSpace(word) || containsNonLatin(word) {
+		if isJunkWord(word) {
 			continue
 		}