feat: Phase 1 – Konten, Kategorien, Transaktionen, EZB-Währungsumrechnung

Backend:
- Modelle + Migration 0002: accounts, account_identifiers, category_groups,
  categories, transactions, exchange_rates (alle owner-gescoped).
- EZB-Kursdienst (90-Tage/Historie) + Backfill-Skript; Umrechnung EUR<->CHF zum
  Buchungsdatum mit lokalem Cache und manueller Übersteuerung; EUR-Pivot
  (amount_eur_cents) eingefroren.
- Kontostände + Netto-Vermögen (Umrechnung in Hauptwährung).
- Router: accounts (inkl. IBAN-Identifikatoren), categories (Gruppen+Kategorien),
  transactions (Ausgabe/Einnahme/Transfer, cross-currency), rates.
- Gruppierte Standard-Kategorien je Nutzer beim Anlegen.
- pytest-Suite: Währung, Salden, Cross-Currency-Transfer, Nutzer-Isolation.

Frontend:
- Seiten Konten, Kategorien, Buchungen; Icon-Chip, Geldbetrag mit Umrechnung,
  Icon-/Farbwähler, Modal; erweitertes SVG-Icon-Set (Konto-/Kategorie-Icons).
- Währungs-/Datumsformatierung (EUR de-DE, CHF de-CH), i18n DE/EN erweitert.
- Live-Umrechnungs-Vorschau im Transfer-Formular.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/tests/test_accounts.py

@@ -0,0 +1,84 @@
+from datetime import date, timedelta
+from decimal import Decimal
+
+
+def _seed_recent_rates(rate: str = "0.95") -> None:
+    from app.database import SessionLocal
+    from app.models import ExchangeRate
+
+    db = SessionLocal()
+    try:
+        today = date.today()
+        for delta in range(0, 7):
+            day = today - timedelta(days=delta)
+            if db.get(ExchangeRate, (day, "CHF")) is None:
+                db.add(ExchangeRate(date=day, currency="CHF", rate_per_eur=Decimal(rate)))
+        db.commit()
+    finally:
+        db.close()
+
+
+def _mk_account(admin, **kw):
+    body = {"name": "A", "type": "girokonto", "currency": "EUR", "opening_balance_cents": 0}
+    body.update(kw)
+    r = admin.post("/api/accounts", json=body)
+    assert r.status_code == 201, r.text
+    return r.json()
+
+
+def test_starter_categories_present(admin):
+    groups = admin.get("/api/categories").json()
+    assert len(groups) >= 6
+    names = {c["name"] for g in groups for c in g["categories"]}
+    assert {"Gehalt", "Lebensmittel", "Sparen & Investments"} <= names
+
+
+def test_currency_validation(admin):
+    r = admin.post("/api/accounts", json={"name": "X", "currency": "USD"})
+    assert r.status_code == 422
+
+
+def test_identifiers_stored_and_deduped(admin):
+    acc = _mk_account(
+        admin,
+        name="Mit IBAN",
+        identifiers=[
+            {"kind": "iban", "value": "DE12 3456"},
+            {"kind": "iban", "value": "de123456"},  # normiert identisch -> dedupliziert
+            {"kind": "card_no", "value": "1234"},
+        ],
+    )
+    kinds = sorted((i["kind"], i["value"]) for i in acc["identifiers"])
+    assert len(acc["identifiers"]) == 2
+    assert ("card_no", "1234") in kinds
+
+
+def test_balance_and_cross_currency_transfer(admin):
+    _seed_recent_rates("0.95")
+    d = date.today()
+    eur = _mk_account(admin, name="Giro", currency="EUR", opening_balance_cents=100000)
+    chf = _mk_account(admin, name="Spar", type="sparkonto", currency="CHF", opening_balance_cents=0)
+
+    admin.post("/api/transactions", json={"type": "income", "date": str(d), "account_id": eur["id"], "amount_cents": 50000})
+    admin.post("/api/transactions", json={"type": "expense", "date": str(d), "account_id": eur["id"], "amount_cents": 20000})
+    assert admin.get(f"/api/accounts/{eur['id']}").json()["balance_cents"] == 130000
+
+    # Transfer 100 EUR -> CHF (0.95) = 95 CHF gutgeschrieben.
+    r = admin.post("/api/transactions", json={
+        "type": "transfer", "date": str(d), "account_id": eur["id"],
+        "to_account_id": chf["id"], "amount_cents": 10000,
+    })
+    assert r.status_code == 201, r.text
+    assert r.json()["to_amount_cents"] == 9500
+    assert admin.get(f"/api/accounts/{eur['id']}").json()["balance_cents"] == 120000
+    assert admin.get(f"/api/accounts/{chf['id']}").json()["balance_cents"] == 9500
+
+
+def test_transfer_requires_other_account(admin):
+    eur = _mk_account(admin, name="Solo", currency="EUR")
+    d = date.today()
+    r = admin.post("/api/transactions", json={
+        "type": "transfer", "date": str(d), "account_id": eur["id"],
+        "to_account_id": eur["id"], "amount_cents": 100,
+    })
+    assert r.status_code == 400