feat(fp): WP-15 — missing stories: shell + brief components

Add the 7 stories CLAUDE.md's testing rule ("UI is exercised via Storybook
stories") was missing: shared/layout/shell (Design System/Templates/Shell)
and all six previously-unstoried brief components (passage-picker,
rejection-comments, diagnostics-panel, letter-block, letter-preview,
letter-section — Domein/Brief/*), each with a default state plus the
meaningful variants (locked/editable, findings/clean, show/entry, etc).
Every *.component.ts in the repo now has a co-located story; *.page.ts
files stay unstoried, matching the existing norm.

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

src/app/shared/layout/shell/shell.stories.ts

@@ -0,0 +1,16 @@
+import type { Meta, StoryObj } from '@storybook/angular';
+import { applicationConfig } from '@storybook/angular';
+import { provideRouter } from '@angular/router';
+import { ShellComponent } from './shell.component';
+
+const meta: Meta<ShellComponent> = {
+  title: 'Design System/Templates/Shell',
+  component: ShellComponent,
+  decorators: [applicationConfig({ providers: [provideRouter([])] })],
+};
+export default meta;
+type Story = StoryObj<ShellComponent>;
+
+// No route matches, so <router-outlet> renders nothing — this story is about the
+// persistent chrome (skip-link, header, footer), not routed page content.
+export const Default: Story = {};