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)

.github/copilot-instructions.md

@@ -228,6 +228,7 @@ Keep docs synced with code. When you change services/MQTT/API/UTC/env or dev/pro
   Documentation maintenance: keep this file aligned with real patterns; update when routes/session/UTC rules change. Avoid long prose; link exact paths.
 
 ## Frontend patterns (dashboard)
+- **UI design rules**: Component choices, layout structure, button variants, badge colors, dialog patterns, toast conventions, and tab structure are defined in [`FRONTEND_DESIGN_RULES.md`](../FRONTEND_DESIGN_RULES.md). Follow that file for all dashboard work.
 - Vite React app; proxies `/api` and `/screenshots` to API in dev (`vite.config.ts`).
 - Uses Syncfusion components; Vite config pre-bundles specific packages to avoid alias issues.
 - Environment: `VITE_API_URL` provided at build/run; in dev compose, proxy handles `/api` so local fetches can use relative `/api/...` paths.
@@ -235,7 +236,7 @@ Keep docs synced with code. When you change services/MQTT/API/UTC/env or dev/pro
 - **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.
 
 - Dev Container: When adding frontend deps, prefer `npm ci` and, if using named volumes, recreate dashboard `node_modules` volume so installs occur inside the container.
-- Theming: Syncfusion Material 3 theme is used. All component CSS is imported centrally in `dashboard/src/main.tsx` (base, navigations, buttons, inputs, dropdowns, popups, kanban, grids, schedule, filemanager, notifications, layouts, lists, calendars, splitbuttons, icons). Tailwind CSS has been removed.
+- Theming: All Syncfusion component CSS is imported centrally in `dashboard/src/main.tsx`. Theme conventions, component defaults, the full CSS import list, and Tailwind removal are documented in `FRONTEND_DESIGN_RULES.md`.
  - Scheduler (appointments page): top bar includes Group and Academic Period selectors (Syncfusion DropDownList). Selecting a period calls `POST /api/academic_periods/active`, moves the calendar to today’s month/day within the period year, and refreshes a right-aligned indicator row showing:
    - Holidays present in the current view (count)
    - Period label (display_name or name) with a badge indicating whether any holidays exist in that period (overlap check)