feat(splitscreen): add split-screen feature for multi-app side-by-side view

Add new @manacore/shared-splitscreen package enabling iFrame-based
split-screen functionality across Calendar, Todo, and Contacts apps.

Features:
- SplitPaneContainer with CSS Grid layout
- AppPanel with iFrame sandbox permissions and loading/error states
- ResizeHandle with mouse, touch, and keyboard support (20-80% range)
- PanelControls for swap and close actions
- Svelte 5 runes-based store with Context API
- URL persistence (?panel=todo&split=60)
- localStorage persistence with versioning
- Mobile auto-disable (<1024px breakpoint)

Integration:
- PillNavigation: added onOpenInPanel prop and Ctrl/Cmd+click support
- PillDropdown: added split button per app item
- Calendar, Todo, Contacts layouts wrapped with SplitPaneContainer

Also fixes:
- WeekView.svelte: fixed {@const} placement error
- MultiDayView.svelte: fixed {@const} placement error

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/central-services/README.md

@@ -10,6 +10,7 @@ Dieses Verzeichnis dokumentiert zentrale Services, die von allen Manacore-Apps g
 | **Theming** | Theme-Varianten, Dark Mode, Accessibility, Custom Themes | [THEMING.md](./THEMING.md) |
 | **Help** | Zentrale Hilfeseite mit FAQ, Features, Shortcuts, Changelog | [HELP.md](./HELP.md) |
 | **Command Bar** | Globale Schnellsuche und Navigation (Cmd/Ctrl+K) | [COMMAND-BAR.md](./COMMAND-BAR.md) |
+| **Split-Screen** | Zwei Apps nebeneinander im Browser (iFrame-basiert) | [SPLIT-SCREEN.md](./SPLIT-SCREEN.md) |
 
 ## Architektur-Prinzipien
 
@@ -80,6 +81,7 @@ pnpm db:push
 | `@manacore/shared-help-content` | Content-Loader, Parser, Merger, Suche |
 | `@manacore/shared-help-ui` | Svelte UI-Komponenten für Hilfeseite |
 | `@manacore/shared-help-mobile` | React Native Komponenten für Hilfe |
+| `@manacore/shared-splitscreen` | Split-Screen Container, Store, Komponenten |
 
 ## Hinzufügen neuer zentraler Services