refactor(errors): defer mutation errors to global handler

Now that MutationCache toasts via toUserMessage by default, the per-hook
onError duplicates drop away. Also normalizes inline query-error UI and
surfaces a previously-silent failure.

Mutations — removed redundant onError toasts:
- network-selector: useAcceptInvitation, createNetwork (inline)
- network-settings: useInviteMembers, useRevokeInvitation, useRemoveMember
- network-billing: useCreateCheckoutSession, useCreatePortalSession
  (onSuccess toasts stay — they carry domain context like network name)

Queries — consistent inline error UX via toUserMessage:
- network-selector: failed-to-load state gets a "Try again" button
- network-billing: "Couldn't load billing" includes friendly reason
- network-settings: useNetworkInvitations failure now surfaces a hint
  (previously rendered as "0 pending" — silently wrong)

Trimmed noisy JSDoc from PR 1 files (errors.ts, query-client.ts,
app-error-boundary.tsx, error-fallback.tsx, ipc-utils.ts).

js/src/features/network-billing.tsx

@@ -1,6 +1,5 @@
 import { useState } from "react";
 import { ExternalLink } from "lucide-react";
-import { toast } from "sonner";
 import { Badge } from "@/components/ui/badge";
 import { Button } from "@/components/ui/button";
 import { Label } from "@/components/ui/label";
@@ -9,6 +8,7 @@ import { Separator } from "@/components/ui/separator";
 import { Muted } from "@/components/ui/typography";
 import { CopyableEmail } from "@/components/copyable-email";
 import { cn } from "@/lib/utils";
+import { toUserMessage } from "@/lib/errors";
 import { SUPPORT_EMAIL } from "@/lib/constants";
 import {
   useCreateCheckoutSession,
@@ -172,7 +172,6 @@ function FreeBilling({
   const handleUpgrade = () => {
     createCheckout.mutate(cadence, {
       onSuccess: ({ url }) => window.electronLink.openExternal(url),
-      onError: (err) => toast.error(err.message || "Failed to start checkout"),
     });
   };
 
@@ -230,8 +229,6 @@ function ProBilling({
   const handleManage = () => {
     createPortal.mutate(undefined, {
       onSuccess: ({ url }) => window.electronLink.openExternal(url),
-      onError: (err) =>
-        toast.error(err.message || "Failed to open billing portal"),
     });
   };
 
@@ -342,7 +339,7 @@ function AdminBillingControls({ networkId }: { networkId: string }) {
   if (error) {
     return (
       <div className="px-4 py-3">
-        <Muted className="text-sm">Failed to load billing.</Muted>
+        <Muted className="text-sm">Couldn't load billing: {toUserMessage(error)}</Muted>
       </div>
     );
   }

js/src/features/network-selector.tsx

@@ -22,6 +22,7 @@ import { useNetworks } from "@/hooks/use-networks";
 import { useMyInvitations, useAcceptInvitation } from "@/hooks/use-member-management";
 import { apiClient } from "@/api/client";
 import { Progress } from "@/components/ui/progress";
+import { toUserMessage } from "@/lib/errors";
 import type { Network, Invitation } from "@/api/types";
 
 function NetworkRow({
@@ -86,9 +87,6 @@ function InvitationRow({ invitation }: { invitation: Invitation }) {
       onSuccess: () => {
         toast.success(`Joined ${invitation.network_name}`);
       },
-      onError: (err) => {
-        toast.error(err.message || "Failed to accept invitation");
-      },
     });
   };
 
@@ -138,9 +136,6 @@ function CreateNetworkDialog({
       setName("");
       navigate(`/${network.id}/settings`);
     },
-    onError: (err) => {
-      toast.error(err.message || "Failed to create network");
-    },
   });
 
   const handleSubmit = (e: React.FormEvent) => {
@@ -200,9 +195,12 @@ export default function NetworkSelector() {
 
   if (error) {
     return (
-      <div className="flex h-full items-center justify-center">
-        <p className="text-destructive text-sm">Failed to load networks</p>
-        <p>{error.message}</p>
+      <div className="flex h-full flex-col items-center justify-center gap-3 p-6 text-center">
+        <p className="text-sm font-medium">Couldn't load your networks</p>
+        <p className="text-muted-foreground text-xs">{toUserMessage(error)}</p>
+        <Button size="sm" variant="outline" onClick={() => refetch()}>
+          Try again
+        </Button>
       </div>
     );
   }

js/src/features/network-settings.tsx

@@ -79,9 +79,6 @@ function InviteForm({ networkId }: { networkId: string }) {
         toast.success(`Invitation sent to ${trimmed}`);
         setEmail("");
       },
-      onError: (err) => {
-        toast.error(err.message || "Failed to send invitation");
-      },
     });
   };
 
@@ -119,9 +116,6 @@ function PendingInvitationRow({
       onSuccess: () => {
         toast.success(`Invitation to ${email} revoked`);
       },
-      onError: (err) => {
-        toast.error(err.message || "Failed to revoke invitation");
-      },
     });
   };
 
@@ -188,7 +182,7 @@ export default function NetworkSettingsPage() {
   const [searchParams] = useSearchParams();
   const { data: networks } = useNetworks();
   const network = networks?.find((n) => n.id === networkId);
-  const { data: invitations } = useNetworkInvitations(networkId!);
+  const { data: invitations, error: invitationsError } = useNetworkInvitations(networkId!);
   const currentUser = useAuthStore((s) => s.user);
   const isAdmin = currentUser?.id === network?.admin_human.id;
   const [memberToRemove, setMemberToRemove] = useState<Human | null>(null);
@@ -286,6 +280,14 @@ export default function NetworkSettingsPage() {
             />
             <Separator />
             <InviteForm networkId={networkId!} />
+            {invitationsError && (
+              <>
+                <Separator />
+                <p className="text-muted-foreground px-4 py-3 text-xs">
+                  Couldn't load pending invitations.
+                </p>
+              </>
+            )}
             {pendingCount > 0 && (
               <>
                 <Separator />
@@ -354,9 +356,6 @@ export default function NetworkSettingsPage() {
                 toast.success(`Removed ${target.email}`);
                 setMemberToRemove(null);
               },
-              onError: (err) => {
-                toast.error(err.message || "Failed to remove member");
-              },
             });
           }}
           onClose={() => {