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/lib/services/custom-fields.service.ts

@@ -3,7 +3,7 @@ import { and, eq, count } from 'drizzle-orm';
 import { db } from '@/lib/db';
 import { customFieldDefinitions, customFieldValues } from '@/lib/db/schema/system';
 import { createAuditLog, type AuditMeta } from '@/lib/audit';
-import { NotFoundError, ValidationError, ConflictError } from '@/lib/errors';
+import { CodedError, NotFoundError, ValidationError, ConflictError } from '@/lib/errors';
 import type { CreateFieldInput, UpdateFieldInput } from '@/lib/validators/custom-fields';
 import type { CustomFieldDefinition } from '@/lib/db/schema/system';
 
@@ -89,7 +89,10 @@ export async function createDefinition(
     .returning();
 
   const created = rows[0];
-  if (!created) throw new Error('Insert failed - no row returned');
+  if (!created)
+    throw new CodedError('INSERT_RETURNING_EMPTY', {
+      internalMessage: 'Custom field definition insert returned no row',
+    });
 
   void createAuditLog({
     userId,
@@ -141,7 +144,10 @@ export async function updateDefinition(
     .returning();
 
   const updated = updateRows[0];
-  if (!updated) throw new Error('Update failed - no row returned');
+  if (!updated)
+    throw new CodedError('INSERT_RETURNING_EMPTY', {
+      internalMessage: 'Custom field definition update returned no row',
+    });
 
   void createAuditLog({
     userId,