Say which property a JMAP SetError rejected

"Send failed: Invalid property or value." is Stalwart's description for
invalidProperties, and on its own it says nothing about what to fix. The
SetError also carries a `properties` array naming the offending fields, which
every call site was discarding.

setErrorMessage appends them, and the 35 places that surfaced a SetError -
send, save draft, mailboxes, calendars, contacts, sieve, files, signature
images, sharing - now go through it.

web/src/lib/__tests__/seterror.test.ts

@@ -0,0 +1,18 @@
+import { describe, expect, it } from "vitest";
+import { setErrorMessage } from "@/jmap/client";
+
+describe("setErrorMessage", () => {
+  it("names the offending properties, which is usually the whole answer", () => {
+    expect(setErrorMessage({ type: "invalidProperties", description: "Invalid property or value.", properties: ["sentAt"] }))
+      .toBe("Invalid property or value. (sentAt)");
+    expect(setErrorMessage({ type: "invalidProperties", properties: ["to", "cc"] }))
+      .toBe("invalidProperties (to, cc)");
+  });
+
+  it("falls back cleanly when the server says less", () => {
+    expect(setErrorMessage({ type: "forbidden" })).toBe("forbidden");
+    expect(setErrorMessage({ type: "forbidden", description: "Not allowed" })).toBe("Not allowed");
+    expect(setErrorMessage({ type: "x", properties: [] })).toBe("x");
+    expect(setErrorMessage(null)).toBe("Unknown error");
+  });
+});