feat: financial year planning — annual budgets, income tracking, household sharing

- Financial year page (/financial-year): year selector, 3 KPI cards (income,
  fixed costs, actual expenses), income and budget-items tabs with inline CRUD
- Revenue accounts as income source: salary-months toggle (12/13) per account
- Household support: create household, invite members by email (existing and
  new users via PendingHouseholdInvite), accept invitations, set roles
- Combined household income view across all active members
- FinancialYear, YearlyIncome, YearlyBudgetItem, Household, HouseholdMembership
  models with migrations; household invite email template
- Management command to migrate existing accounts/budgets to financial years
- FinancialYearService in Angular with full API integration
- Dashboard updated: income/fixed-costs read from financial year data,
  year dropdown synced with available financial years
- Sidebar: financial year nav item added
- i18n: all keys in DE/EN/FR/IT

backend/core/settings.py

@@ -104,6 +104,7 @@ EMAIL_HOST_USER     = os.environ.get('EMAIL_HOST_USER', '')
 EMAIL_HOST_PASSWORD = os.environ.get('EMAIL_HOST_PASSWORD', '')
 EMAIL_USE_TLS       = os.environ.get('EMAIL_USE_TLS', 'True') == 'True'
 DEFAULT_FROM_EMAIL  = os.environ.get('DEFAULT_FROM_EMAIL', 'noreply@armarium.ch')
+FRONTEND_URL        = os.environ.get('FRONTEND_URL', 'http://localhost:4200')
 
 LOGGING = {
     'version': 1,