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/settings/SettingsPage.tsx

@@ -24,6 +24,7 @@ interface SubscriptionInfo {
   trialEndsAt: string | null;
   currentPeriodEnd: string | null;
   cancelAtPeriodEnd: boolean;
+  hasStripeCustomer: boolean;
 }
 
 const statusColors: Record<string, string> = {
@@ -68,8 +69,9 @@ export function SettingsPage() {
       if (data.url) {
         window.location.href = data.url;
       }
-    } catch {
-      notifications.show({ message: 'Unable to open billing portal', color: 'red' });
+    } catch (err: any) {
+      const msg = err.response?.data?.message || 'Unable to open billing portal';
+      notifications.show({ message: typeof msg === 'string' ? msg : 'Unable to open billing portal', color: 'red' });
     } finally {
       setPortalLoading(false);
     }
@@ -167,17 +169,23 @@ export function SettingsPage() {
                   <Badge variant="light" color="cyan" size="sm">Invoice / ACH</Badge>
                 </Group>
               )}
-              <Button
-                variant="light"
-                color="teal"
-                size="sm"
-                leftSection={<IconCreditCard size={16} />}
-                onClick={handleManageBilling}
-                loading={portalLoading}
-                mt="xs"
-              >
-                Manage Billing
-              </Button>
+              {subscription.hasStripeCustomer ? (
+                <Button
+                  variant="light"
+                  color="teal"
+                  size="sm"
+                  leftSection={<IconCreditCard size={16} />}
+                  onClick={handleManageBilling}
+                  loading={portalLoading}
+                  mt="xs"
+                >
+                  Manage Billing
+                </Button>
+              ) : subscription.status === 'trial' ? (
+                <Text size="xs" c="dimmed" mt="xs">
+                  Billing portal will be available once you add a payment method.
+                </Text>
+              ) : null}
             </Stack>
           ) : (
             <Text size="sm" c="dimmed">No active subscription</Text>