T8: ISO 4217 per-currency decimals + format_money()

- Add CURRENCY_DECIMALS dict (JPY=0, BHD/KWD/OMR=3, rest=2) and DEFAULT_CURRENCY_DECIMALS=2
- Add CurrencyConverter.format_money() staticmethod with ROUND_HALF_UP; symbol-prefix for known, code-prefix for unknown
- Replace 14 :,.2f literals across services.py / info.py / config_cmd.py with format_money() calls
- Refactor 2 i18n templates (transaction_add.ok, recurring_add.ok): {amount:.2f} -> {amount_str}; callers pass pre-formatted string
- New _account_currency() helper for transaction currency lookup
- Extend test_currency.py: 8 new tests (JPY/BHD/USD/CHF/negative/zero/float-int/lowercase)
- 75/75 tests pass; 0 :,.2f literals remain in infrastructure/cli/

infrastructure/cli/services.py

@@ -54,7 +54,11 @@ def convert_value(
     return converter.convert(amount, from_curr, to_curr)
 
 
-def fmt_amount(amount: Decimal | float, currency: str, symbol: str) -> str:
-    if isinstance(amount, Decimal):
-        return f"{symbol}{amount:,.2f}"
-    return f"{symbol}{amount:,.2f}"
+def fmt_amount(amount: Decimal | float, currency: str, symbol: str = "") -> str:
+    """Format money via the canonical display rule.
+
+    `symbol` is retained as a positional parameter for backward compatibility with
+    existing callers (compare.py / forecast.py / scenario.py / whatif.py); the
+    symbol/layout decision is centralised in `CurrencyConverter.format_money`.
+    """
+    return CurrencyConverter.format_money(amount, currency)