fix: billing portal error, onboarding wizard improvements, budget empty state

- Fix "Manage Billing" button error for trial orgs without Stripe customer;
  add fallback to retrieve customer from subscription, show helpful message
  for trial users, and surface real error messages in the UI
- Add "Balance As-Of Date" field to onboarding wizard so opening balance
  journal entries use the correct statement date instead of today
- Add "Total Unit Count" field to onboarding wizard assessment group step
  so cash flow projections work immediately
- Remove broken budget upload step from onboarding wizard (was using legacy
  budgets endpoint); replace with guidance to use Budget Planning page
- Replace bare "No budget plan lines" text with rich onboarding-style card
  featuring download template and upload CSV action buttons

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/modules/accounts/dto/create-account.dto.ts

@@ -37,6 +37,11 @@ export class CreateAccountDto {
   @IsOptional()
   initialBalance?: number;
 
+  @ApiProperty({ required: false, description: 'ISO date string (YYYY-MM-DD) for when the initial balance was accurate' })
+  @IsString()
+  @IsOptional()
+  initialBalanceDate?: string;
+
   @ApiProperty({ required: false, description: 'Annual interest rate as a percentage' })
   @IsOptional()
   interestRate?: number;