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/ai-budget.service.ts

@@ -15,6 +15,7 @@ import { and, eq, gte, sql } from 'drizzle-orm';
 import { db } from '@/lib/db';
 import { aiUsageLedger } from '@/lib/db/schema/ai-usage';
 import { systemSettings } from '@/lib/db/schema/system';
+import { ValidationError } from '@/lib/errors';
 import { logger } from '@/lib/logger';
 
 export type BudgetPeriod = 'day' | 'week' | 'month';
@@ -70,10 +71,10 @@ export async function setAiBudget(
     period: input.period ?? existing.period,
   };
   if (next.softCapTokens < 0 || next.hardCapTokens < 0) {
-    throw new Error('Token caps must be non-negative');
+    throw new ValidationError('Token caps must be non-negative');
   }
   if (next.softCapTokens > next.hardCapTokens) {
-    throw new Error('softCapTokens cannot exceed hardCapTokens');
+    throw new ValidationError('softCapTokens cannot exceed hardCapTokens');
   }
   await db
     .delete(systemSettings)