fix: quantity/price fields padded with zeros when editing a transaction

The API returns quantity and price_per_unit at their full Numeric(38,18)
scale (e.g. "3.000000000000000000") since crypto quantities need up to 18
decimal places for wei-level amounts — correct for computation, but the
edit form pre-filled these raw strings straight into the inputs.

Added trimTrailingZeros() to lib/format.ts and used it when pre-filling
CryptoTransactionModal in edit mode. Trims the string directly rather than
round-tripping through Number(), which would risk rounding a genuine
18-decimal-place amount through floating point instead of just stripping
padding.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01855PiW5Wd8n4aRW4n9kByY

frontend/src/components/crypto/CryptoTransactionModal.tsx

@@ -4,6 +4,7 @@ import { Button } from "@/components/ui/Button";
 import { Dialog } from "@/components/ui/Dialog";
 import { Input, Label } from "@/components/ui/Input";
 import { useAddCryptoTransaction, useUpdateCryptoTransaction } from "@/hooks/useCrypto";
+import { trimTrailingZeros } from "@/lib/format";
 import { useTranslation } from "@/lib/i18n";
 import { cn } from "@/lib/utils";
 import type { CryptoHolding, CryptoTransaction, CryptoTransactionType } from "@/types";
@@ -39,8 +40,11 @@ export function CryptoTransactionModal({ open, onClose, holding, transaction = n
     if (!open) return;
     if (transaction) {
       setType(transaction.type);
-      setQuantity(transaction.quantity);
+      // The API returns these at full Numeric(38,18) scale (e.g.
-      setPricePerUnit(transaction.price_per_unit);
+      // "3.000000000000000000") — trimmed here so the field shows what was
+      // actually entered, not a wall of padding zeros.
+      setQuantity(trimTrailingZeros(transaction.quantity));
+      setPricePerUnit(trimTrailingZeros(transaction.price_per_unit));
       setDate(transaction.date);
       setNote(transaction.note ?? "");
     } else {

frontend/src/lib/format.ts

@@ -32,6 +32,20 @@ export function maskAmount(formatted: string, hidden: boolean): string {
   return hidden ? "••••" : formatted;
 }
 
+/** Strips trailing zeros from a decimal string for pre-filling an editable
+ * number input — the backend stores crypto quantity/price as Numeric(38,18)
+ * and returns it at full scale (e.g. "3.000000000000000000"), which is
+ * correct for computation but not something anyone wants to see or edit
+ * around in a form field. Works on the string directly rather than via
+ * Number(), which would round a genuine 18-decimal-place amount (a
+ * wei-level token quantity) through floating point instead of just
+ * trimming the padding. */
+export function trimTrailingZeros(value: string): string {
+  if (!value.includes(".")) return value;
+  const trimmed = value.replace(/0+$/, "").replace(/\.$/, "");
+  return trimmed === "" || trimmed === "-" ? "0" : trimmed;
+}
+
 const MONTH_LABELS_EN = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"] as const;
 const MONTH_LABELS_RU = ["Янв", "Фев", "Мар", "Апр", "Май", "Июн", "Июл", "Авг", "Сен", "Окт", "Ноя", "Дек"] as const;