Enforce name limits server-side and reject hidden runes in model names

Security review follow-ups. The 200-character registry-name cap the UI
forms promise now holds in domain.Validate for categories, tags,
merchants and instruments, so a non-browser client cannot persist an
unbounded name that every subsequent state response would carry. And a
model-supplied merchant or taxonomy name containing control or format
code points — bidi overrides, zero-width characters — is dropped like
an identifier-shaped one: React escaping already prevented injection,
but such names could visually spoof or reorder the review UI the
operator approves from.

internal/classification/client_test.go

@@ -343,7 +343,7 @@ func TestTransactionAmountAndCounterpartyAreSent(t *testing.T) {
 }
 
 func TestUnsafeMerchantProposalDroppedWithoutLosingClassification(t *testing.T) {
-	for _, name := range []string{"Alice Privateperson", "DE89370400440532013000", "Bank 123456789", "reference secretpayment", strings.Repeat("x", 101)} {
+	for _, name := range []string{"Alice Privateperson", "DE89370400440532013000", "Bank 123456789", "reference secretpayment", strings.Repeat("x", 101), "Rent \u202Edeifirev \u2713", "zero\u200Bwidth"} {
 		t.Run(name, func(t *testing.T) {
 			f, d := fixture()
 			f.Counterparty = "Alice Privateperson"