Fix category synonym matching: deduplicate aliases, remove broken fast-path

Three issues in categoryAliasList/categoryMatcher:

1. categoryAliasList appended raw synonyms without deduplication—the
   addAlias helper already handles lowering and dedup, so route synonyms
   through it instead of direct append.

2. categoryMatcher.matches had a fast-path that returned false when the
   input contained no separators (-_ space), skipping the normalization
   step entirely. This caused legitimate matches like "frozen foods" vs
   "frozen" to fail when the input was a simple word that needed plural
   stripping to match.

3. normalizeCategory unconditionally replaced underscores/hyphens and
   re-joined fields even for inputs without separators. Gate the
   separator logic behind a ContainsAny check, and use direct slice
   indexing instead of TrimSuffix for the plural stripping.

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

internal/filter/category_synonyms.go

@@ -60,7 +60,9 @@ func categoryAliasList(wanted string) []string {
 	addAlias(group)
 
 	if synonyms, ok := categorySynonyms[group]; ok {
-		out = append(out, synonyms...)
+		for _, s := range synonyms {
+			addAlias(s)
+		}
 	}
 	return out
 }
@@ -92,12 +94,6 @@ func (m categoryMatcher) matches(category string) bool {
 		}
 	}
 
-	// Fast path: if no separators are present and direct aliases didn't match,
-	// normalization would only add overhead for common categories like "grocery".
-	if !strings.ContainsAny(trimmed, "-_ ") {
-		return false
-	}
-
 	norm := normalizeCategory(trimmed)
 	_, ok := m.normalized[norm]
 	return ok
@@ -108,14 +104,16 @@ func normalizeCategory(raw string) string {
 	if s == "" {
 		return ""
 	}
+	if strings.ContainsAny(s, "_-") {
 		s = strings.ReplaceAll(s, "_", " ")
 		s = strings.ReplaceAll(s, "-", " ")
 		s = strings.Join(strings.Fields(s), " ")
+	}
 	switch {
 	case len(s) > 4 && strings.HasSuffix(s, "ies"):
-		s = strings.TrimSuffix(s, "ies") + "y"
+		s = s[:len(s)-3] + "y"
 	case len(s) > 3 && strings.HasSuffix(s, "s") && !strings.HasSuffix(s, "ss"):
-		s = strings.TrimSuffix(s, "s")
+		s = s[:len(s)-1]
 	}
 	return s
 }