Add dev-only state debug view (Elm-style "show the Model")

A floating, read-only panel that renders the current root-store state
(SessionStore + BigProfileStore) live via the json pipe. Whole thing is
gated by isDevMode() so it never renders or ships in production.

- Observer only — no new store/library/state pattern (PRD prime directive).
- BigProfileStore resolved lazily on first open; its httpResources fetch
  eagerly on construction, so we avoid a personal-data fetch on pages that
  don't use it.
- bsn masked before render; no persistence/logging/network in the feature.
- maskBsn has a unit spec; UI exercised via a Devtools Storybook story,
  per repo convention (no TestBed component tests).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/shared/ui/debug-state/debug-state.component.ts

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+import { Component, Injector, computed, inject, isDevMode, signal } from '@angular/core';
+import { JsonPipe } from '@angular/common';
+import { SessionStore } from '@auth/application/session.store';
+import { Session } from '@auth/domain/session';
+import { BigProfileStore } from '@registratie/application/big-profile.store';
+import { maskBsn } from './mask';
+
+/**
+ * Dev-only "show the current Model" panel (Elm-debugger style, read-only).
+ * Observes the root singletons and renders them via the json pipe. Never a
+ * product feature: the whole template is gated by isDevMode(), so it does not
+ * render and is unreachable in a production build.
+ *
+ * ponytail: dev tool — intentionally off-theme (not Rijkshuisstijl) and exempt
+ * from the design-token convention; plain values keep it visually distinct.
+ */
+@Component({
+  selector: 'app-debug-state',
+  imports: [JsonPipe],
+  styles: [`
+    .fab{position:fixed;bottom:1rem;right:1rem;z-index:9999;font:600 12px/1 monospace;
+      background:#1e1e1e;color:#9cdcfe;border:1px solid #444;border-radius:4px;
+      padding:.5rem .75rem;cursor:pointer}
+    .panel{position:fixed;bottom:3.25rem;right:1rem;z-index:9999;width:min(90vw,28rem);
+      max-height:70vh;overflow:auto;background:#1e1e1e;color:#d4d4d4;border:1px solid #444;
+      border-radius:6px;box-shadow:0 6px 24px rgba(0,0,0,.4)}
+    .panel pre{margin:0;padding:.75rem;font:12px/1.5 monospace;white-space:pre-wrap;
+      word-break:break-word}
+  `],
+  template: `
+    @if (isDev) {
+      <button type="button" class="fab" (click)="toggle()">
+        {{ visible() ? '× state' : '⚙ state' }}
+      </button>
+      @if (visible()) {
+        <div class="panel"><pre>{{ snapshot() | json }}</pre></div>
+      }
+    }
+  `,
+})
+export class DebugStateComponent {
+  protected readonly isDev = isDevMode();
+  protected readonly visible = signal(false);
+
+  private session = inject(SessionStore);
+  private injector = inject(Injector);
+  // Resolved on first open only — BigProfileStore's httpResources fetch eagerly
+  // on construction, so we must not instantiate it until the dev asks to look.
+  private profileStore?: BigProfileStore;
+
+  protected readonly snapshot = computed(() => ({
+    session: maskSession(this.session.session()),
+    profile: this.profileStore?.profile(),
+    decisions: this.profileStore?.decisions(),
+    aantekeningen: this.profileStore?.aantekeningen(),
+    pendingHerregistratie: this.profileStore?.pendingHerregistratie(),
+  }));
+
+  toggle(): void {
+    if (!this.visible()) this.profileStore ??= this.injector.get(BigProfileStore);
+    this.visible.update((v) => !v);
+  }
+}
+
+function maskSession(s: Session | null): Session | null {
+  return s ? { ...s, bsn: maskBsn(s.bsn) } : null;
+}

src/app/shared/ui/debug-state/debug-state.stories.ts

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+import type { Meta, StoryObj } from '@storybook/angular';
+import { DebugStateComponent } from './debug-state.component';
+
+// Devtools, not a design-system atom — titled accordingly.
+const meta: Meta<DebugStateComponent> = {
+  title: 'Devtools/State debug',
+  component: DebugStateComponent,
+};
+export default meta;
+type Story = StoryObj<DebugStateComponent>;
+
+// The floating button; click it to open the read-only state panel. Stores
+// resolve to their default (empty/loading) state here.
+export const Default: Story = {};

src/app/shared/ui/debug-state/mask.spec.ts

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+import { describe, it, expect } from 'vitest';
+import { maskBsn } from './mask';
+
+describe('maskBsn', () => {
+  it('keeps the last 3 digits and masks the rest', () => {
+    expect(maskBsn('123456789')).toBe('******789');
+  });
+
+  it('handles short and empty input without throwing', () => {
+    expect(maskBsn('12')).toBe('**');
+    expect(maskBsn('')).toBe('');
+  });
+});

src/app/shared/ui/debug-state/mask.ts

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+/** Redact a BSN for the dev state view: keep the last 3 digits, mask the rest. */
+export function maskBsn(bsn: string): string {
+  if (bsn.length <= 3) return '*'.repeat(bsn.length);
+  return '*'.repeat(bsn.length - 3) + bsn.slice(-3);
+}