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/

domain/currency.py

@@ -13,6 +13,32 @@ CURRENCY_SYMBOLS = {
     "UAH": "₴",
 }
 
+# ISO 4217 minor units per currency (https://en.wikipedia.org/wiki/ISO_4217).
+# Keys are uppercase 3-letter codes, values are the number of decimal digits used
+# when DISPLAYING the currency. Internal arithmetic keeps full Decimal precision
+# (do not modify — see T8 decision `internal_precision`).
+#
+# - 0 decimals: JPY (Japanese yen)
+# - 2 decimals: most currencies (RUB, USD, EUR, GBP, CNY, KZT, UAH, ...)
+# - 3 decimals: BHD (Bahraini dinar), KWD (Kuwaiti dinar), OMR (Omani rial)
+#
+# Unknown currencies fall back to `DEFAULT_CURRENCY_DECIMALS` (2).
+CURRENCY_DECIMALS: dict[str, int] = {
+    "JPY": 0,
+    "RUB": 2,
+    "USD": 2,
+    "EUR": 2,
+    "GBP": 2,
+    "CNY": 2,
+    "KZT": 2,
+    "UAH": 2,
+    "BHD": 3,
+    "KWD": 3,
+    "OMR": 3,
+}
+
+DEFAULT_CURRENCY_DECIMALS = 2
+
 
 class ExchangeRate(BaseModel):
     from_currency: str = "USD"
@@ -78,6 +104,31 @@ class CurrencyConverter:
     def get_symbol(self, currency: str) -> str:
         return CURRENCY_SYMBOLS.get(currency, currency)
 
+    @staticmethod
+    def format_money(
+        amount: Decimal | float | int | str,
+        currency: str,
+    ) -> str:
+        """Format `amount` for display using ISO 4217 decimal digits for `currency`.
+
+        Rounding: ROUND_HALF_UP (0.5 -> 1) before display — internal arithmetic in
+        Decimal is unchanged. Known symbols are rendered as a tight prefix
+        (e.g. "$1,234.57"); unknown codes are rendered as a spaced prefix
+        ("BHD 1.234") so the layout is consistent regardless of symbol availability.
+        Negative values render with the sign before the symbol ("-₽1,200.00").
+        """
+        cur = currency.upper()
+        decimals = CURRENCY_DECIMALS.get(cur, DEFAULT_CURRENCY_DECIMALS)
+        a = Decimal(str(amount)) if not isinstance(amount, Decimal) else amount
+        quantizer = Decimal("1") if decimals == 0 else Decimal(1).scaleb(-decimals)
+        quantized = a.quantize(quantizer, rounding=ROUND_HALF_UP)
+        sign = "-" if quantized < 0 else ""
+        abs_quantized = -quantized if quantized < 0 else quantized
+        num_str = f"{abs_quantized:,.{decimals}f}"
+        if cur in CURRENCY_SYMBOLS:
+            return f"{sign}{CURRENCY_SYMBOLS[cur]}{num_str}"
+        return f"{sign}{cur} {num_str}"
+
     @classmethod
     def with_defaults(cls) -> "CurrencyConverter":
         return cls(DEFAULT_RATES)