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/templates/emails/household_invite.html

@@ -0,0 +1,34 @@
+{% extends "emails/base.html" %}
+
+{% block subject %}Armarium – Einladung zum Haushalt{% endblock %}
+
+{% block body %}
+  <p style="margin:0 0 20px;font-size:15px;color:#374151;line-height:1.6;">Hallo {{ invitee_name }},</p>
+
+  <p style="margin:0 0 20px;font-size:15px;color:#374151;line-height:1.6;">
+    <strong>{{ inviter_name }}</strong> hat dich eingeladen, dem Haushalt
+    <strong>{{ household_name }}</strong> auf Armarium beizutreten.
+  </p>
+
+  <table role="presentation" width="100%" cellpadding="0" cellspacing="0" style="margin:0 0 28px;">
+    <tr>
+      <td align="center">
+        <a href="{{ accept_url }}"
+           style="display:inline-block;background-color:#7c3aed;color:#ffffff;font-size:15px;font-weight:600;text-decoration:none;padding:14px 32px;border-radius:8px;letter-spacing:0.1px;">
+          {{ cta_label }}
+        </a>
+      </td>
+    </tr>
+  </table>
+
+  <p style="margin:0 0 8px;font-size:13px;color:#6b7280;line-height:1.6;">
+    Falls der Button nicht funktioniert, kopiere diesen Link in deinen Browser:
+  </p>
+  <p style="margin:0 0 24px;font-size:13px;color:#7c3aed;line-height:1.6;word-break:break-all;">
+    {{ accept_url }}
+  </p>
+
+  <p style="margin:0;font-size:15px;color:#374151;line-height:1.6;">
+    – Das Armarium-Team
+  </p>
+{% endblock %}

backend/templates/emails/household_invite.txt

@@ -0,0 +1,10 @@
+Hallo {{ invitee_name }},
+
+{{ inviter_name }} hat dich eingeladen, dem Haushalt "{{ household_name }}" auf Armarium beizutreten.
+
+{{ cta_label }}:
+{{ accept_url }}
+
+Falls du diese Einladung nicht erwartet hast, kannst du sie ignorieren.
+
+– Das Armarium-Team