chore: remove orphaned backoffice/builder dead code

Sprint 2 high-priority cleanup: backoffice-dashboard.component.ts and
builder-sandbox.component.ts were not routed anywhere, superseded by
features/backoffice and features/project-editor. Their sole facades
(BackofficeFacade, BuilderConfigFacade) had no other consumers.

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

src/app/facades/backoffice/backoffice.facade.ts

@@ -1,40 +0,0 @@
-import { Injectable, signal } from '@angular/core';
-import { ProductCardConfig, CategoryCardConfig } from '../../shared/models/ui';
-import { BackofficeDataService } from '../../core/backoffice/backoffice-data.service';
-
-@Injectable({ providedIn: 'root' })
-export class BackofficeFacade {
-  readonly products = signal<ProductCardConfig[]>([]);
-  readonly categories = signal<CategoryCardConfig[]>([]);
-  readonly loading = signal(false);
-
-  constructor(private readonly backofficeDataService: BackofficeDataService) {}
-
-  load(): void {
-    this.loading.set(true);
-
-    this.backofficeDataService.loadProducts().subscribe({
-      next: (products) => {
-        this.products.set(products ?? []);
-        this.loadCategories();
-      },
-      error: () => {
-        this.products.set([]);
-        this.loadCategories();
-      }
-    });
-  }
-
-  private loadCategories(): void {
-    this.backofficeDataService.loadCategories().subscribe({
-      next: (categories) => {
-        this.categories.set(categories ?? []);
-        this.loading.set(false);
-      },
-      error: () => {
-        this.categories.set([]);
-        this.loading.set(false);
-      }
-    });
-  }
-}

src/app/facades/builder/builder-config.facade.ts

@@ -1,52 +0,0 @@
-import { Injectable, computed, signal } from '@angular/core';
-import { take } from 'rxjs/operators';
-import { BootstrapConfig } from '../../shared/models/config';
-import { ConfigService } from '../../core/config/config.service';
-
-@Injectable({ providedIn: 'root' })
-export class BuilderConfigFacade {
-  private readonly bootstrapState = signal<BootstrapConfig | null>(null);
-
-  readonly bootstrap = this.bootstrapState.asReadonly();
-  readonly branding = computed(() => this.bootstrapState()?.branding ?? null);
-  readonly featureFlags = computed(() => this.bootstrapState()?.featureFlags ?? null);
-
-  constructor(private readonly configService: ConfigService) {}
-
-  load(): void {
-    this.configService.loadBootstrap().pipe(take(1)).subscribe({
-      next: (config) => this.bootstrapState.set(JSON.parse(JSON.stringify(config)) as BootstrapConfig),
-      error: () => this.bootstrapState.set(null)
-    });
-  }
-
-  updateBrandName(nextBrandName: string): void {
-    const current = this.bootstrapState();
-    if (!current) {
-      return;
-    }
-
-    this.bootstrapState.set({
-      ...current,
-      branding: {
-        ...current.branding,
-        brandName: nextBrandName
-      }
-    });
-  }
-
-  setFeatureFlag(flag: string, enabled: boolean): void {
-    const current = this.bootstrapState();
-    if (!current) {
-      return;
-    }
-
-    this.bootstrapState.set({
-      ...current,
-      featureFlags: {
-        ...current.featureFlags,
-        [flag]: enabled
-      }
-    });
-  }
-}