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)

README.md

@@ -388,6 +388,8 @@ mosquitto_sub -h localhost -t "infoscreen/+/heartbeat" -v
 
 ## 🎨 Frontend Features
 
+> UI implementation conventions (component choices, layout, buttons, dialogs, badge colors, toast patterns, locale) are documented in [FRONTEND_DESIGN_RULES.md](FRONTEND_DESIGN_RULES.md).
+
 ### API Response Format
 - **JSON Convention**: All API endpoints return camelCase JSON (e.g., `startTime`, `endTime`, `groupId`). Frontend consumes camelCase directly.
 - **UTC Time Parsing**: API returns ISO strings without 'Z' suffix. Frontend appends 'Z' before parsing to ensure UTC interpretation: `const utcString = dateStr.endsWith('Z') ? dateStr : dateStr + 'Z'; new Date(utcString);`. Display uses `toLocaleTimeString('de-DE')` for German format.