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/accounts.service.ts

@@ -74,9 +74,9 @@ export class AccountsService {
 
     // Create opening balance journal entry if initialBalance is provided and non-zero
     if (dto.initialBalance && dto.initialBalance !== 0) {
-      const now = new Date();
-      const year = now.getFullYear();
-      const month = now.getMonth() + 1;
+      const balanceDate = dto.initialBalanceDate ? new Date(dto.initialBalanceDate) : new Date();
+      const year = balanceDate.getFullYear();
+      const month = balanceDate.getMonth() + 1;
 
       // Find the current fiscal period
       const periods = await this.tenant.query(
@@ -111,12 +111,14 @@ export class AccountsService {
           );
         }
 
-        // Create the journal entry
+        // Create the journal entry (use provided balance date or today)
+        const entryDate = dto.initialBalanceDate || new Date().toISOString().split('T')[0];
         const jeInsert = await this.tenant.query(
           `INSERT INTO journal_entries (entry_date, description, entry_type, fiscal_period_id, is_posted, posted_at, created_by)
-           VALUES (CURRENT_DATE, $1, 'opening_balance', $2, true, NOW(), $3)
+           VALUES ($1::date, $2, 'opening_balance', $3, true, NOW(), $4)
            RETURNING id`,
           [
+            entryDate,
             `Opening balance for ${dto.name}`,
             fiscalPeriodId,
             '00000000-0000-0000-0000-000000000000',