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>

packages/shared-splitscreen/src/utils/index.ts

@@ -0,0 +1,13 @@
+/**
+ * Split-Screen Utilities
+ * Re-export all utility functions.
+ */
+
+export { parseUrlState, updateUrlState, clearUrlState, getCurrentUrlState } from './url-state.js';
+
+export {
+	savePanelState,
+	loadPanelState,
+	clearPanelState,
+	createStorageConfig,
+} from './local-storage.js';