phase-9: add builder sandbox for in-memory configuration editing

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

@@ -0,0 +1,52 @@
+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
+      }
+    });
+  }
+}