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>

frontend/src/pages/board-planning/BudgetPlanningPage.tsx

@@ -1,13 +1,13 @@
 import { useState, useEffect, useRef } from 'react';
 import {
   Title, Table, Group, Button, Stack, Text, NumberInput,
-  Select, Loader, Center, Badge, Card, Alert, Modal,
+  Select, Loader, Center, Badge, Card, Alert, Modal, ThemeIcon,
 } from '@mantine/core';
 import { notifications } from '@mantine/notifications';
 import {
   IconDeviceFloppy, IconInfoCircle, IconPencil, IconX,
   IconCheck, IconArrowBack, IconTrash, IconRefresh,
-  IconUpload, IconDownload,
+  IconUpload, IconDownload, IconFileSpreadsheet,
 } from '@tabler/icons-react';
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
 import api from '../../services/api';
@@ -659,7 +659,37 @@ export function BudgetPlanningPage() {
                 {lineData.length === 0 && (
                   <Table.Tr>
                     <Table.Td colSpan={15}>
-                      <Text ta="center" c="dimmed" py="lg">No budget plan lines.</Text>
+                      <Card withBorder p="xl" mx="auto" maw={600} my="lg" style={{ textAlign: 'center' }}>
+                        <ThemeIcon size={60} radius="xl" variant="light" color="blue" mx="auto" mb="md">
+                          <IconFileSpreadsheet size={28} />
+                        </ThemeIcon>
+                        <Title order={4} mb="xs">Get Started with Your {selectedYear} Budget</Title>
+                        <Text c="dimmed" size="sm" mb="lg" maw={450} mx="auto">
+                          Your budget plan is created but has no line items yet. Download the
+                          CSV template pre-filled with your chart of accounts, fill in your
+                          monthly amounts, then upload it here.
+                        </Text>
+                        <Group justify="center" gap="md">
+                          <Button
+                            variant="light"
+                            leftSection={<IconDownload size={16} />}
+                            onClick={handleDownloadTemplate}
+                          >
+                            Download Budget Template
+                          </Button>
+                          <Button
+                            leftSection={<IconUpload size={16} />}
+                            onClick={handleImportCSV}
+                            loading={importMutation.isPending}
+                          >
+                            Upload Budget CSV
+                          </Button>
+                        </Group>
+                        <Text size="xs" c="dimmed" mt="md">
+                          Tip: The template includes all your active accounts. Fill in the monthly
+                          dollar amounts for each line, save as CSV, then upload.
+                        </Text>
+                      </Card>
                     </Table.Td>
                   </Table.Tr>
                 )}