Add fee rate unit preference & dropdown

frontend/src/app/components/rate-unit-selector/rate-unit-selector.component.html

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+<div [formGroup]="rateUnitForm" class="text-small text-center">
+    <select formControlName="rateUnits" class="custom-select custom-select-sm form-control-secondary form-control mx-auto" style="width: 200px;" (change)="changeUnits()">
+        <option *ngFor="let unit of units" [value]="unit.name">{{ unit.label }}</option>
+    </select>
+</div>

frontend/src/app/components/rate-unit-selector/rate-unit-selector.component.ts

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+import { ChangeDetectionStrategy, Component, OnDestroy, OnInit } from '@angular/core';
+import { UntypedFormBuilder, UntypedFormGroup } from '@angular/forms';
+import { StorageService } from '../../services/storage.service';
+import { StateService } from '../../services/state.service';
+import { Subscription } from 'rxjs';
+
+@Component({
+  selector: 'app-rate-unit-selector',
+  templateUrl: './rate-unit-selector.component.html',
+  styleUrls: ['./rate-unit-selector.component.scss'],
+  changeDetection: ChangeDetectionStrategy.OnPush
+})
+export class RateUnitSelectorComponent implements OnInit, OnDestroy {
+  rateUnitForm: UntypedFormGroup;
+  rateUnitSub: Subscription;
+  units = [
+    { name: 'vb', label: 'sat/vB' },
+    { name: 'wu', label: 'sat/WU' },
+  ];
+
+  constructor(
+    private formBuilder: UntypedFormBuilder,
+    private stateService: StateService,
+    private storageService: StorageService,
+  ) { }
+
+  ngOnInit() {
+    this.rateUnitForm = this.formBuilder.group({
+      rateUnits: ['vb']
+    });
+    this.rateUnitSub = this.stateService.rateUnits$.subscribe((units) => {
+      this.rateUnitForm.get('rateUnits')?.setValue(units);
+    });
+  }
+
+  changeUnits() {
+    const newUnits = this.rateUnitForm.get('rateUnits')?.value;
+    this.storageService.setValue('rate-unit-preference', newUnits);
+    this.stateService.rateUnits$.next(newUnits);
+  }
+
+  ngOnDestroy(): void {
+    this.rateUnitSub.unsubscribe();
+  }
+}