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/admin/shared/settings-form-card.tsx

@@ -18,6 +18,7 @@ import {
   SelectValue,
 } from '@/components/ui/select';
 import { apiFetch } from '@/lib/api/client';
+import { toastError } from '@/lib/api/toast-error';
 
 export type SettingFieldType =
   | 'string'
@@ -116,7 +117,7 @@ export function SettingsFormCard({ title, description, fields, extra }: Settings
       toast.success(`Saved ${changedFields.length} setting(s)`);
       setOriginals(values);
     } catch (err) {
-      toast.error(err instanceof Error ? err.message : 'Save failed');
+      toastError(err);
     } finally {
       setSaving(false);
     }