feat(cibg): adopt the vendored CIBG Bestand-upload component

The upload suite now wraps the vendored CIBG file-upload classes instead of
hand-rolling from tokens:
- file-input becomes a .file-picker-drop-area with drag-and-drop + a real
  .btn-upload button and a visually-hidden, focusable <input>; renders an
  always-visible instruction (allowed types + max size) linked to the input via
  aria-describedby (pattern requirement).
- The file list is a semantic ul.file-list; single-upload is an
  li[app-single-upload] .file-container (native <li> child) with .actions
  (retry/.icon-remove) + progress; document-chip renders the .file block
  (status glyph, .file-name link, .file-meta size/status).
- Category validation moves ABOVE the block as .upload-validation > .feedback.
- Instruction text uses the default (not subtle) foreground for WCAG AA contrast
  on the grey drop-area.

DocumentUpload's public inputs/outputs are unchanged — both wizards keep working.
GREEN: lint, tokens, 181 tests, build, 137 axe stories.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app/shared/ui/upload/file-input/file-input.stories.ts

@@ -6,16 +6,20 @@ const meta: Meta<FileInputComponent> = {
   component: FileInputComponent,
   render: (args) => ({
     props: args,
-    template: `<app-file-input [inputId]="inputId" [accept]="accept" [multiple]="multiple" [disabled]="disabled" />`,
+    template: `<app-file-input [inputId]="inputId" [accept]="accept" [maxSizeMb]="maxSizeMb" [multiple]="multiple" [disabled]="disabled" />`,
   }),
 };
 export default meta;
 type Story = StoryObj<FileInputComponent>;
 
 export const Default: Story = {
-  args: { inputId: 'diploma', accept: ['application/pdf'], multiple: false, disabled: false },
+  args: { inputId: 'diploma', accept: ['application/pdf', 'image/jpeg'], maxSizeMb: 10, multiple: false, disabled: false },
+};
+
+export const Multiple: Story = {
+  args: { inputId: 'cv', accept: ['application/pdf'], maxSizeMb: 5, multiple: true, disabled: false },
 };
 
 export const Disabled: Story = {
-  args: { inputId: 'diploma-disabled', accept: ['application/pdf'], multiple: false, disabled: true },
+  args: { inputId: 'diploma-disabled', accept: ['application/pdf'], maxSizeMb: 10, multiple: false, disabled: true },
 };