Fix clients group filter and search for non-ASCII capitals (#6685)

* Match non-ASCII capitals in the clients group filter and search

SQLite LOWER() only folds ASCII, so a group or search term with a Cyrillic,
Persian or other non-ASCII capital never matched after being lower-cased in
Go. Also compare the value as typed.

* Match lower, upper and title-case spellings for non-ASCII search and group filter

internal/web/service/client_paging_test.go

@@ -635,3 +635,36 @@ func TestListPagedEmptyPanel(t *testing.T) {
 		t.Fatal("groups = nil, want an empty list so the filter drawer renders")
 	}
 }
+
+// SQLite's LOWER() folds ASCII only, so non-ASCII capitals must still match.
+func TestListPagedNonASCIICase(t *testing.T) {
+	svc, inboundSvc, settingSvc := setupPagingServices(t)
+	rec := model.ClientRecord{Email: "lima@x", Comment: "Привет", Group: "Тест", Enable: true}
+	if err := database.GetDB().Create(&rec).Error; err != nil {
+		t.Fatalf("create client: %v", err)
+	}
+	for name, params := range map[string]ClientPageParams{
+		"group":             {PageSize: 50, Group: "Тест"},
+		"group other case":  {PageSize: 50, Group: "ТЕСТ"},
+		"search":            {PageSize: 50, Search: "Привет"},
+		"search lower case": {PageSize: 50, Search: "привет"},
+		"search upper case": {PageSize: 50, Search: "ПРИВЕТ"},
+	} {
+		resp, err := svc.ListPaged(inboundSvc, settingSvc, params)
+		if err != nil {
+			t.Fatalf("%s: ListPaged: %v", name, err)
+		}
+		if got := pagedEmails(resp.Items); !slices.Equal(got, []string{"lima@x"}) {
+			t.Fatalf("%s: emails = %v, want [lima@x]", name, got)
+		}
+	}
+}
+
+func TestCaseVariants(t *testing.T) {
+	if got, want := caseVariants("пРИвет"), []string{"привет", "пРИвет", "ПРИВЕТ", "Привет"}; !slices.Equal(got, want) {
+		t.Fatalf("caseVariants = %v, want %v", got, want)
+	}
+	if got := caseVariants("abc"); !slices.Equal(got, []string{"abc", "ABC", "Abc"}) {
+		t.Fatalf("ASCII variants = %v", got)
+	}
+}