fix(label): refuse duplicate label names unless --force

tea label create --name bug twice created a second "bug" label, after
which issue edit --add-labels bug failed as ambiguous and issue list
--label bug matched nothing. label create (and --file imports) now fail
with "label "bug" already exists in alice/project; pass --force to
update it", ignoring case; --force updates the existing label. The
ambiguity error for labels that already share a name lists their IDs.

Signed-off-by: bircni <bircni@icloud.com>

internal/app/labelfile_test.go

@@ -51,6 +51,10 @@ func TestLabelImportAcceptsColorPrefixedRows(t *testing.T) {
 	var created []string
 	server := newAuthenticatedServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		require.Equal(t, "/api/v1/repos/alice/project/labels", r.URL.Path)
+		if r.Method == http.MethodGet {
+			_, _ = w.Write([]byte(`[]`))
+			return
+		}
 		payload, err := io.ReadAll(r.Body)
 		require.NoError(t, err)
 		var label struct{ Name string }