feat(story-dialog): conform aan dialog-pattern + AlertDialog delete

Story 4 van PBI "Alle dialogen conform docs/patterns/dialog.md".

- lib/schemas/story.ts — gedeeld zod-schema
- actions/stories.ts — code+fieldErrors voor 422; code: 403 voor auth/demo
- StoryDialog adopt useDirtyCloseGuard, useDialogSubmitShortcut,
  entityDialog* layout-classes
- Inline delete-confirm vervangen door AlertDialog (§10.4)
- docs/specs/dialogs/story.md — gaps weggewerkt; alleen bewuste
  afwijkingen blijven (header met badges, geen char-counter)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/schemas/story.ts

@@ -0,0 +1,26 @@
+import { z } from 'zod'
+import { MAX_CODE_LENGTH } from '@/lib/code'
+
+const codeField = z.string().max(MAX_CODE_LENGTH).optional()
+
+export const createStorySchema = z.object({
+  pbiId: z.string(),
+  productId: z.string(),
+  code: codeField,
+  title: z.string().min(1, 'Titel is verplicht').max(200),
+  description: z.string().max(2000).optional(),
+  acceptance_criteria: z.string().max(2000).optional(),
+  priority: z.coerce.number().int().min(1).max(4),
+})
+
+export const updateStorySchema = z.object({
+  id: z.string(),
+  code: codeField,
+  title: z.string().min(1, 'Titel is verplicht').max(200),
+  description: z.string().max(2000).optional(),
+  acceptance_criteria: z.string().max(2000).optional(),
+  priority: z.coerce.number().int().min(1).max(4),
+})
+
+export type CreateStoryInput = z.infer<typeof createStorySchema>
+export type UpdateStoryInput = z.infer<typeof updateStorySchema>