feat(pbi-dialog): conform aan dialog-pattern + DemoTooltip + dirty-guard

Story 3 van PBI "Alle dialogen conform docs/patterns/dialog.md".

- lib/schemas/pbi.ts — gedeeld zod-schema (createPbiSchema/updatePbiSchema)
- actions/pbis.ts — returnen nu code+fieldErrors (422) en code: 403 voor
  auth/demo errors
- PbiDialog adopt useDirtyCloseGuard, useDialogSubmitShortcut,
  entityDialog* layout-classes; submit-knop + Annuleren in DemoTooltip
- isDemo-prop toegevoegd, pbi-list geeft 'm door
- docs/specs/dialogs/pbi.md — "Bekende gaps" weggewerkt; alleen bewuste
  uitsluitingen blijven

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

lib/schemas/pbi.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()
+const statusField = z.enum(['ready', 'blocked', 'done']).optional()
+
+export const createPbiSchema = z.object({
+  productId: z.string(),
+  code: codeField,
+  title: z.string().min(1, 'Titel is verplicht').max(200),
+  description: z.string().max(2000).optional(),
+  priority: z.coerce.number().int().min(1).max(4),
+  status: statusField,
+})
+
+export const updatePbiSchema = z.object({
+  id: z.string(),
+  code: codeField,
+  title: z.string().min(1, 'Titel is verplicht').max(200),
+  description: z.string().max(2000).optional(),
+  priority: z.coerce.number().int().min(1).max(4),
+  status: statusField,
+})
+
+export type CreatePbiInput = z.infer<typeof createPbiSchema>
+export type UpdatePbiInput = z.infer<typeof updatePbiSchema>