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/email-compose.service.ts

@@ -6,7 +6,7 @@ import { emailAccounts, emailMessages, emailThreads } from '@/lib/db/schema/emai
 import { documents, documentEvents, files } from '@/lib/db/schema/documents';
 import { createAuditLog, type AuditMeta } from '@/lib/audit';
 import { env } from '@/lib/env';
-import { NotFoundError, ForbiddenError } from '@/lib/errors';
+import { CodedError, NotFoundError, ForbiddenError } from '@/lib/errors';
 import { logger } from '@/lib/logger';
 import { getDecryptedCredentials } from '@/lib/services/email-accounts.service';
 import { getPortEmailConfig } from '@/lib/services/port-config';
@@ -195,7 +195,10 @@ export async function sendEmail(
       })
       .returning();
     const newThread = newThreadRows[0];
-    if (!newThread) throw new Error('Failed to create email thread');
+    if (!newThread)
+      throw new CodedError('INSERT_RETURNING_EMPTY', {
+        internalMessage: 'Failed to create email thread',
+      });
     threadId = newThread.id;
   }
 
@@ -222,7 +225,10 @@ export async function sendEmail(
     .returning();
 
   const message = messageRows[0];
-  if (!message) throw new Error('Failed to persist outbound email message');
+  if (!message)
+    throw new CodedError('INSERT_RETURNING_EMPTY', {
+      internalMessage: 'Failed to persist outbound email message',
+    });
 
   // Update thread metadata
   await db