fix(api): read the error code before the status

A read-failure primitive decided from the status alone, so a 401 from a
step-up or re-proof gate would have read as an ended session and sent
the reader to sign-in. The code now decides first and the status is the
fallback for codes it does not know.

The two invitation routes answer an unknown name or e-mail with
sharing.invite.no_account. Both have a second 404 with a different next
step, and since every error now carries a code the clients match on it
instead of on the status. The status defaults move into a module the
client can import, and the contract text no longer says these arms
carry no code.

src/lib/api/default-error-codes.ts

@@ -0,0 +1,45 @@
+/**
+ * The `meta.errorCode` an error envelope carries when its route named none.
+ *
+ * `meta.errorCode` is what a client is told to branch on, and roughly half of
+ * the refusals in the tree carried one: the rest answered prose alone, so a
+ * client could branch on the code for one 404 and had to fall back to the
+ * status for the next. Every error envelope now has a code. A route that names
+ * its own keeps it; one that does not gets the status's code from this table,
+ * so the generic family is predictable from the status alone and a client
+ * never has to special-case an absent field.
+ *
+ * Deliberately coarse. These say which kind of refusal it was, never why —
+ * a route that has a reason worth branching on names its own code. Statuses
+ * not listed fall back to `http.client_error` / `http.server_error`.
+ */
+export const DEFAULT_ERROR_CODES = {
+  400: "http.bad_request",
+  401: "http.unauthenticated",
+  403: "http.forbidden",
+  404: "http.not_found",
+  405: "http.method_not_allowed",
+  409: "http.conflict",
+  410: "http.gone",
+  413: "http.payload_too_large",
+  415: "http.unsupported_media_type",
+  422: "http.invalid",
+  429: "http.rate_limited",
+  500: "http.internal",
+  502: "http.bad_gateway",
+  503: "http.unavailable",
+  504: "http.gateway_timeout",
+  client: "http.client_error",
+  server: "http.server_error",
+} as const;
+
+/** The default code for an error status, see {@link DEFAULT_ERROR_CODES}. */
+export function defaultErrorCodeForStatus(status: number): string {
+  const listed = (DEFAULT_ERROR_CODES as Record<number, string | undefined>)[
+    status
+  ];
+  if (listed) return listed;
+  return status >= 500
+    ? DEFAULT_ERROR_CODES.server
+    : DEFAULT_ERROR_CODES.client;
+}