feat: client-side currency conversion with admin-configurable rates

- CurrencyRatesService: RUB-based rates, persisted via localStorage
- CurrencyConvertPipe: impure pipe converting item price to selected currency
- Admin settings page: editable currency rates form
- Applied conversion to product-card, product-information, quick-view-dialog,
  delivery-information, compare-table, cart totals + payment payload

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/services/currency-rates.service.ts

@@ -0,0 +1,53 @@
+import { Injectable, Signal, inject, signal } from '@angular/core';
+import { LocalStorageService } from '../core/storage/local-storage.service';
+
+const RATES_KEY = 'currencyRates.v1';
+
+/** Fallback rates relative to RUB (1 RUB = rate[code] units of code), used until admin overrides them. */
+const DEFAULT_RATES: Record<string, number> = {
+  RUB: 1,
+  USD: 0.011,
+  EUR: 0.01,
+  AMD: 4.3,
+};
+
+@Injectable({ providedIn: 'root' })
+export class CurrencyRatesService {
+  private readonly localStorage = inject(LocalStorageService);
+
+  private readonly ratesSignal = signal<Record<string, number>>(this.readStoredRates());
+
+  readonly rates: Signal<Record<string, number>> = this.ratesSignal.asReadonly();
+
+  /** Base currency all rates are relative to. */
+  readonly baseCurrency = 'RUB';
+
+  getRate(code: string): number {
+    return this.ratesSignal()[code] ?? DEFAULT_RATES[code] ?? 1;
+  }
+
+  setRate(code: string, rate: number): void {
+    if (!Number.isFinite(rate) || rate <= 0) {
+      return;
+    }
+    const next = { ...this.ratesSignal(), [code]: rate };
+    this.ratesSignal.set(next);
+    this.localStorage.setJSON(RATES_KEY, next);
+  }
+
+  /** Converts an amount expressed in `fromCurrency` into `toCurrency` via the RUB base rate. */
+  convert(amount: number, fromCurrency: string, toCurrency: string): number {
+    if (fromCurrency === toCurrency) {
+      return amount;
+    }
+    const fromRate = this.getRate(fromCurrency);
+    const toRate = this.getRate(toCurrency);
+    const amountInBase = amount / fromRate;
+    return amountInBase * toRate;
+  }
+
+  private readStoredRates(): Record<string, number> {
+    const stored = this.localStorage.getJSON<Record<string, number>>(RATES_KEY);
+    return { ...DEFAULT_RATES, ...stored };
+  }
+}