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/jmap/client.ts

@@ -346,3 +346,17 @@ export function chunk<T>(arr: T[], size: number): T[][] {
   for (let i = 0; i < arr.length; i += size) out.push(arr.slice(i, i + size));
   return out;
 }
+
+/**
+ * A readable message for a JMAP SetError.
+ *
+ * Servers name the offending field in `properties`, which is usually the whole
+ * answer to "why was this rejected" — Stalwart's description alone is often
+ * just "Invalid property or value." Keep both.
+ */
+export function setErrorMessage(err: { type: string; description?: string; properties?: string[] } | null | undefined): string {
+  if (!err) return "Unknown error";
+  const base = err.description ?? err.type;
+  const props = err.properties?.length ? ` (${err.properties.join(", ")})` : "";
+  return `${base}${props}`;
+}