fix(audit-tier-2): error-surface hygiene — toastError + CodedError sweep

Two mechanical sweeps closing the audit's HIGH §16 + MED §11 findings:

* 38 client components / 56 toast.error sites converted to
  toastError(err) so the new admin error inspector becomes usable from
  user-reported issues — every failed inline-edit, save, send, archive,
  upload, etc. now carries the request-id + error-code (Copy ID action).
* 26 service files / 62 bare-Error throws converted to CodedError or
  the existing AppError subclasses.  Adds new error codes:
  DOCUMENSO_UPSTREAM_ERROR (502), DOCUMENSO_AUTH_FAILURE (502),
  DOCUMENSO_TIMEOUT (504), OCR_UPSTREAM_ERROR (502),
  IMAP_UPSTREAM_ERROR (502), UMAMI_UPSTREAM_ERROR (502),
  UMAMI_NOT_CONFIGURED (409), and INSERT_RETURNING_EMPTY (500) for
  post-insert returning-empty guards.
* Five vitest assertions updated to match the new user-facing wording
  (client-merge "already been merged", expense/interest "couldn't find
  that …", documenso "signing service didn't respond").

Test status: 1168/1168 vitest, tsc clean.

Refs: docs/audit-comprehensive-2026-05-05.md HIGH §16 (auditor-H Issue 1)
+ MED §11 (auditor-G Issue 1).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/notifications/notification-preferences-form.tsx

@@ -7,6 +7,7 @@ import { toast } from 'sonner';
 import { Button } from '@/components/ui/button';
 import { Switch } from '@/components/ui/switch';
 import { apiFetch } from '@/lib/api/client';
+import { toastError } from '@/lib/api/toast-error';
 
 interface Pref {
   notificationType: string;
@@ -72,7 +73,7 @@ export function NotificationPreferencesForm() {
       toast.success('Preferences saved');
       qc.invalidateQueries({ queryKey: ['notifications', 'preferences'] });
     },
-    onError: (err) => toast.error(err instanceof Error ? err.message : 'Save failed'),
+    onError: (err) => toastError(err),
   });
 
   function update(type: string, field: 'inApp' | 'email', value: boolean) {

src/components/notifications/reminder-digest-form.tsx

@@ -17,6 +17,7 @@ import {
   SelectValue,
 } from '@/components/ui/select';
 import { apiFetch } from '@/lib/api/client';
+import { toastError } from '@/lib/api/toast-error';
 
 interface ReminderPrefs {
   delivery: 'immediate' | 'daily' | 'weekly' | 'off';
@@ -81,7 +82,7 @@ export function ReminderDigestForm() {
       toast.success('Reminder preferences saved');
       qc.invalidateQueries({ queryKey: ['user', 'preferences'] });
     },
-    onError: (err) => toast.error(err instanceof Error ? err.message : 'Save failed'),
+    onError: (err) => toastError(err),
   });
 
   if (isLoading) {