feat(TrainingStats): enhance training statistics view with collapsible panels and localization

- Refactored the TrainingStatsView to implement collapsible sections for better organization of training statistics.
- Added new localization keys for training statistics panels in both German and English.
- Updated the mobile app's TrainingStatsScreen to utilize the new collapsible panel structure, improving user experience.
- Enhanced the MembersManager to merge training statistics into member data, providing a comprehensive view of member participation.
- Introduced new API methods for quick updates and transfers of member data, streamlining member management processes.

mobile-app/TODO.md

@@ -212,7 +212,6 @@ Web-Route: `/calendar` · Referenz: `CalendarView.vue` (Aggregation mehrerer Dat
 
 ### Phase 12 – Backlog / offen
 
-- [ ] **iOS:** Kalender-UI + Tab (derzeit nur Android `composeApp` / `MainTab.Calendar`)
 - [ ] **i18n:** Kalender-Keys in `MobileStrings.kt` für alle unterstützten Sprachen ergänzen (nicht nur Fallback im Code)
 - [ ] **Kalender vs. Web:** Offizielle Teilnahmen mobil per Browser vs. Web in-app – bewusst lassen oder später angleichen
 - [ ] **Release:** Bei `isMinifyEnabled = true` ProGuard/R8 für Ktor, `kotlinx.serialization`, ggf. Koin (`composeApp/proguard-rules.pro`)