docs: extract frontend design rules and add presentation persistence fix

Create FRONTEND_DESIGN_RULES.md as the single source of truth for all dashboard
UI conventions, including component library (Syncfusion first), component defaults
table, layout structure, buttons, dialogs, badges, toasts, form fields, tabs,
statistics cards, warnings, color palette, CSS files, loading states, locale
rules, and icon conventions (TentTree for skip-holidays events).

Move embedded design rules from ACADEMIC_PERIODS_CRUD_BUILD_PLAN.md to the new
file and replace with a reference link for maintainability.

Update copilot-instructions.md to point to FRONTEND_DESIGN_RULES.md and remove
redundant Syncfusion/Tailwind prose from the Theming section.

Add reference blockquote to README.md under Frontend Features directing readers
to FRONTEND_DESIGN_RULES.md.

Bug fix: Presentation events now reliably persist page_progress and auto_progress
flags across create, update, and detached occurrence flows so display settings
survive round-trips to the API.

Files changed:
- Created: FRONTEND_DESIGN_RULES.md (15 sections, 340+ lines)
- Modified: ACADEMIC_PERIODS_CRUD_BUILD_PLAN.md (extract rules, consolidate)
- Modified: .github/copilot-instructions.md (link to new rules file)
- Modified: README.md (reference blockquote)

dashboard/src/components/CustomSelectUploadEventModal.tsx

@@ -28,6 +28,7 @@ const CustomSelectUploadEventModal: React.FC<CustomSelectUploadEventModalProps>
     path: string;
     name: string;
   } | null>(null);
+  const [selectionError, setSelectionError] = useState<string>('');
 
   // Callback für Dateiauswahl
   interface FileSelectEventArgs {
@@ -42,6 +43,7 @@ const CustomSelectUploadEventModal: React.FC<CustomSelectUploadEventModalProps>
   const handleFileSelect = async (args: FileSelectEventArgs) => {
     if (args.fileDetails.isFile && args.fileDetails.size > 0) {
       const filename = args.fileDetails.name;
+      setSelectionError('');
 
       try {
         const response = await fetch(
@@ -51,10 +53,13 @@ const CustomSelectUploadEventModal: React.FC<CustomSelectUploadEventModalProps>
           const data = await response.json();
           setSelectedFile({ id: data.id, path: data.file_path, name: filename });
         } else {
-          setSelectedFile({ id: filename, path: filename, name: filename });
+          setSelectedFile(null);
+          setSelectionError('Datei ist noch nicht als Medium registriert. Bitte erneut hochladen oder Metadaten prüfen.');
         }
       } catch (e) {
         console.error('Error fetching file details:', e);
+        setSelectedFile(null);
+        setSelectionError('Medium-ID konnte nicht geladen werden. Bitte erneut versuchen.');
       }
     }
   };
@@ -135,6 +140,9 @@ const CustomSelectUploadEventModal: React.FC<CustomSelectUploadEventModalProps>
       >
         <Inject services={[NavigationPane, DetailsView, Toolbar]} />
       </FileManagerComponent>
+      {selectionError && (
+        <div style={{ marginTop: 10, color: '#b71c1c', fontSize: 13 }}>{selectionError}</div>
+      )}
     </DialogComponent>
   );
 };