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/brief/ui/letter-section/letter-section.stories.ts

@@ -0,0 +1,55 @@
+import type { Meta, StoryObj } from '@storybook/angular';
+import { LetterSection, LibraryPassage } from '@brief/domain/brief';
+import { LetterSectionComponent } from './letter-section.component';
+
+const section: LetterSection = {
+  sectionKey: 'kern',
+  title: 'Kern van het besluit',
+  required: true,
+  locked: false,
+  blocks: [
+    {
+      type: 'freeText',
+      blockId: 'local-2',
+      content: {
+        paragraphs: [
+          {
+            nodes: [
+              { type: 'text', text: 'Wij hebben besloten om reden ' },
+              { type: 'placeholder', key: 'reden_besluit' },
+              { type: 'text', text: '.' },
+            ],
+          },
+        ],
+      },
+    },
+  ],
+};
+
+const emptySection: LetterSection = { ...section, blocks: [] };
+
+const passages: LibraryPassage[] = [
+  {
+    passageId: 'p2',
+    scope: 'beroep',
+    beroep: 'arts',
+    sectionKey: 'kern',
+    label: 'Toelichting arts',
+    version: 1,
+    content: { paragraphs: [{ nodes: [{ type: 'text', text: 'Als arts ...' }] }] },
+  },
+];
+
+const placeholders = [{ key: 'reden_besluit', label: 'Reden besluit' }];
+
+const meta: Meta<LetterSectionComponent> = {
+  title: 'Domein/Brief/Letter Section',
+  component: LetterSectionComponent,
+  args: { section, availablePassages: passages, placeholders, edit: () => {} },
+};
+export default meta;
+type Story = StoryObj<LetterSectionComponent>;
+
+export const ReadOnly: Story = { args: { editable: false } };
+export const Editable: Story = { args: { editable: true } };
+export const EditableEmpty: Story = { args: { section: emptySection, editable: true } };