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>

web/src/locales/en.json

@@ -81,5 +81,75 @@
   "server": {
     "unreachable": "Server unreachable.",
     "sessionKept": "Your session is still valid – once the server responds again, it will continue."
+  },
+  "accounts": {
+    "title": "Accounts",
+    "new": "New account",
+    "edit": "Edit account",
+    "name": "Name",
+    "type": "Type",
+    "currency": "Currency",
+    "icon": "Icon",
+    "color": "Color",
+    "openingBalance": "Opening balance",
+    "openingDate": "As of",
+    "identifiers": "Identifiers (IBAN etc.)",
+    "addIdentifier": "Add identifier",
+    "identShort": "id(s)",
+    "total": "Total",
+    "empty": "No accounts yet. Create your first account.",
+    "deleteConfirm": "Delete this account and all its transactions?"
+  },
+  "accountTypes": {
+    "girokonto": "Checking",
+    "sparkonto": "Savings",
+    "bargeld": "Cash",
+    "kreditkarte": "Credit card",
+    "depot": "Brokerage / Investment",
+    "sonstiges": "Other"
+  },
+  "identifierKinds": {
+    "iban": "IBAN",
+    "account_no": "Account number",
+    "card_no": "Card number"
+  },
+  "categories": {
+    "title": "Categories",
+    "newGroup": "New group",
+    "editGroup": "Edit group",
+    "newCategory": "New category",
+    "editCategory": "Edit category",
+    "name": "Name",
+    "kind": "Kind",
+    "income": "Income",
+    "expense": "Expense",
+    "emptyGroup": "No categories in this group.",
+    "deleteConfirm": "Delete this category?",
+    "deleteGroupConfirm": "Delete this group and all its categories?"
+  },
+  "transactions": {
+    "title": "Transactions",
+    "new": "New transaction",
+    "edit": "Edit transaction",
+    "expense": "Expense",
+    "income": "Income",
+    "transfer": "Transfer",
+    "date": "Date",
+    "account": "Account",
+    "fromAccount": "From account",
+    "toAccount": "To account",
+    "category": "Category",
+    "amount": "Amount",
+    "toAmount": "Credited",
+    "payee": "Payee",
+    "description": "Description",
+    "uncategorized": "Uncategorized",
+    "empty": "No transactions yet.",
+    "needAccount": "Create an account first to record transactions.",
+    "allAccounts": "All accounts",
+    "allTypes": "All types",
+    "deleteConfirm": "Delete this transaction?",
+    "amountRequired": "Please enter an amount.",
+    "accountRequired": "Please choose an account."
   }
 }