fix: address cashbox-export whole-branch review findings

- Reject dates that are not strict YYYY-MM-DD (was accepting full ISO
  datetimes, which silently produced empty exports instead of a 400)
  and reject from > to with a 400 before touching the team/DB.
- Emit the cashbox_export_download and cashbox_export_subscription_update
  audit log events that were declared but never fired, matching the
  audit trail every sibling feature already has.
- Restore full type checking on the pdfkit import via `import = require()`
  instead of an untyped require() with an eslint-disable.
- Tighten a cashbox.spec.ts assertion to check the exact dialog class
  instead of expect.anything(), so it can't pass with the wrong dialog
  wired to the Export button.
- Style and announce the export dialogs' error messages using this
  codebase's established error-message/role=alert pattern.

myteamwallet_backend/src/cashbox-export/cashbox-export.utils.ts

@@ -1,6 +1,5 @@
 import { Team } from 'src/teams/entities/team.entity';
-// eslint-disable-next-line @typescript-eslint/no-var-requires
-const PDFDocument = require('pdfkit');
+import PDFDocument = require('pdfkit');
 
 export interface CashboxExportRow {
   date: string;