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/layout/shell/shell.component.ts

@@ -2,12 +2,13 @@ import { Component } from '@angular/core';
 import { RouterOutlet } from '@angular/router';
 import { SiteHeaderComponent } from '@shared/layout/site-header/site-header.component';
 import { SiteFooterComponent } from '@shared/layout/site-footer/site-footer.component';
+import { DebugStateComponent } from '@shared/ui/debug-state/debug-state.component';
 
 /** Template: persistent app chrome. Header + footer mount once; only the routed
     content inside <router-outlet> changes (and cross-fades — see styles.scss). */
 @Component({
   selector: 'app-shell',
-  imports: [RouterOutlet, SiteHeaderComponent, SiteFooterComponent],
+  imports: [RouterOutlet, SiteHeaderComponent, SiteFooterComponent, DebugStateComponent],
   styles: [`
     :host{display:block}
     .skip{position:absolute;left:var(--app-skip-link-offset)}
@@ -26,6 +27,7 @@ import { SiteFooterComponent } from '@shared/layout/site-footer/site-footer.comp
       </main>
       <app-site-footer />
     </div>
+    <app-debug-state />
   `,
 })
 export class ShellComponent {}