feat(codes): UI toont en accepteert code voor taken

- TaskDialog: code-input boven titel (font-mono, optional,
  placeholder "auto"), CodeBadge in dialog header bij edit
- EditTaskLoader: select code uit DB voor de dialog
- Solo page: vervang inline deriveTaskCode-logica door directe
  task.code uit DB
- Sprint page + TaskList + SprintBoardClient: Task-type krijgt
  verplicht code-veld; TaskList laat ongebruikte storyCode prop
  vallen omdat code-derivatie niet meer nodig is

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

app/_components/tasks/task-dialog.tsx

@@ -27,6 +27,8 @@ import {
 } from '@/components/ui/alert-dialog'
 import { Button } from '@/components/ui/button'
 import { Input } from '@/components/ui/input'
+import { CodeBadge } from '@/components/shared/code-badge'
+import { MAX_CODE_LENGTH } from '@/lib/code'
 import { DemoTooltip } from '@/components/shared/demo-tooltip'
 import {
   useDirtyCloseGuard,
@@ -45,6 +47,7 @@ import { cn } from '@/lib/utils'
 
 export interface TaskDialogTask {
   id: string
+  code: string | null
   title: string
   description: string | null
   implementation_plan: string | null
@@ -88,6 +91,7 @@ export function TaskDialog({ task, storyId, productId, closePath, isDemo = false
     resolver: zodResolver(taskSchema),
     mode: 'onTouched',
     defaultValues: {
+      code: task?.code ?? '',
       title: task?.title ?? '',
       description: task?.description ?? '',
       implementation_plan: task?.implementation_plan ?? '',
@@ -173,9 +177,12 @@ export function TaskDialog({ task, storyId, productId, closePath, isDemo = false
         >
           {/* Sticky header */}
           <div className={entityDialogHeaderClasses}>
-            <DialogTitle className="text-xl font-semibold">
-              {isEdit ? 'Taak bewerken' : 'Nieuwe taak'}
-            </DialogTitle>
+            <div className="flex items-center gap-2">
+              <DialogTitle className="text-xl font-semibold">
+                {isEdit ? 'Taak bewerken' : 'Nieuwe taak'}
+              </DialogTitle>
+              {isEdit && task?.code && <CodeBadge code={task.code} />}
+            </div>
             {isEdit && (
               <span className="text-xs text-muted-foreground">
                 Aangemaakt:{' '}
@@ -190,6 +197,27 @@ export function TaskDialog({ task, storyId, productId, closePath, isDemo = false
 
           {/* Scrollable form body */}
           <div className={entityDialogBodyClasses}>
+            {/* Code */}
+            <div>
+              <label htmlFor="task-code" className="text-sm font-medium mb-2 block">
+                Code
+              </label>
+              <Input
+                id="task-code"
+                {...form.register('code')}
+                aria-invalid={!!form.formState.errors.code}
+                placeholder="auto (T-1, T-2, ...)"
+                className="font-mono"
+                maxLength={MAX_CODE_LENGTH}
+                onKeyDown={(e) => { if (e.key === 'Enter') e.preventDefault() }}
+              />
+              {form.formState.errors.code && (
+                <p className="text-xs text-destructive mt-1">
+                  {form.formState.errors.code.message}
+                </p>
+              )}
+            </div>
+
             {/* Title */}
             <div>
               <label className="text-sm font-medium mb-2 block">