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/index.ts

@@ -0,0 +1,46 @@
+/**
+ * @manacore/shared-splitscreen
+ *
+ * Split-screen panel system for ManaCore apps.
+ * Enables displaying two apps side-by-side using iFrames.
+ */
+
+// Types
+export type {
+	PanelConfig,
+	SplitScreenState,
+	AppDefinition,
+	PanelEvent,
+	StorageConfig,
+	UrlState,
+} from './types.js';
+
+export { DIVIDER_CONSTRAINTS, MOBILE_BREAKPOINT } from './types.js';
+
+// Store
+export {
+	createSplitPanelStore,
+	setSplitPanelContext,
+	getSplitPanelContext,
+	hasSplitPanelContext,
+	DEFAULT_APPS,
+	type SplitPanelStore,
+} from './stores/split-panel.svelte.js';
+
+// Utils
+export {
+	parseUrlState,
+	updateUrlState,
+	clearUrlState,
+	getCurrentUrlState,
+	savePanelState,
+	loadPanelState,
+	clearPanelState,
+	createStorageConfig,
+} from './utils/index.js';
+
+// Components (will be added)
+export { default as SplitPaneContainer } from './components/SplitPaneContainer.svelte';
+export { default as AppPanel } from './components/AppPanel.svelte';
+export { default as PanelControls } from './components/PanelControls.svelte';
+export { default as ResizeHandle } from './components/ResizeHandle.svelte';