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/features/admin/settings/pages/admin-settings-page.component.html

@@ -11,4 +11,25 @@
       <span>{{ 'adminSettings.densityCompact' | translate }}</span>
     </label>
   </div>
+
+  <div class="settings-card">
+    <h2>{{ 'adminSettings.currencyRates' | translate }}</h2>
+    <p class="settings-explain">{{ 'adminSettings.currencyRatesExplain' | translate }}</p>
+    <div class="rate-row" *ngFor="let currency of languageService.currencies">
+      <span class="rate-code">{{ currency.code }}</span>
+      <input
+        class="rate-input"
+        type="number"
+        min="0"
+        step="0.0001"
+        [disabled]="currency.code === currencyRates.baseCurrency"
+        [ngModel]="rateDrafts()[currency.code]"
+        (ngModelChange)="onRateInput(currency.code, $event)"
+      />
+    </div>
+    <div class="rate-actions">
+      <button type="button" class="save-button" (click)="saveRates()">{{ 'adminSettings.currencyRatesSave' | translate }}</button>
+      <span class="saved-message" *ngIf="showSavedMessage()">{{ 'adminSettings.currencyRatesSaved' | translate }}</span>
+    </div>
+  </div>
 </section>