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';

packages/shared-splitscreen/src/utils/local-storage.ts

@@ -0,0 +1,97 @@
+/**
+ * LocalStorage Utilities
+ * Handle persistent storage for split-screen preferences.
+ */
+
+import type { SplitScreenState, StorageConfig } from '../types.js';
+import { DIVIDER_CONSTRAINTS } from '../types.js';
+
+const STORAGE_VERSION = 1;
+
+interface StoredState {
+	version: number;
+	state: Partial<SplitScreenState>;
+}
+
+/**
+ * Generate storage key for an app.
+ */
+function getStorageKey(config: StorageConfig): string {
+	return `${config.prefix}-splitscreen-${config.currentAppId}`;
+}
+
+/**
+ * Save split-screen state to localStorage.
+ */
+export function savePanelState(config: StorageConfig, state: Partial<SplitScreenState>): void {
+	if (typeof window === 'undefined') return;
+
+	try {
+		const stored: StoredState = {
+			version: STORAGE_VERSION,
+			state: {
+				dividerPosition: state.dividerPosition,
+				rightPanel: state.rightPanel,
+			},
+		};
+		localStorage.setItem(getStorageKey(config), JSON.stringify(stored));
+	} catch (_error) {
+		// localStorage not available or quota exceeded
+	}
+}
+
+/**
+ * Load split-screen state from localStorage.
+ */
+export function loadPanelState(config: StorageConfig): Partial<SplitScreenState> | null {
+	if (typeof window === 'undefined') return null;
+
+	try {
+		const raw = localStorage.getItem(getStorageKey(config));
+		if (!raw) return null;
+
+		const stored: StoredState = JSON.parse(raw);
+
+		// Version check for future migrations
+		if (stored.version !== STORAGE_VERSION) {
+			clearPanelState(config);
+			return null;
+		}
+
+		// Validate divider position
+		if (stored.state.dividerPosition !== undefined) {
+			stored.state.dividerPosition = Math.max(
+				DIVIDER_CONSTRAINTS.MIN,
+				Math.min(DIVIDER_CONSTRAINTS.MAX, stored.state.dividerPosition)
+			);
+		}
+
+		return stored.state;
+	} catch (_error) {
+		// localStorage not available or corrupted data
+		return null;
+	}
+}
+
+/**
+ * Clear split-screen state from localStorage.
+ */
+export function clearPanelState(config: StorageConfig): void {
+	if (typeof window === 'undefined') return;
+
+	try {
+		localStorage.removeItem(getStorageKey(config));
+	} catch (_error) {
+		// localStorage not available
+	}
+}
+
+/**
+ * Get default storage config with manacore prefix.
+ */
+export function createStorageConfig(currentAppId: string): StorageConfig {
+	return {
+		prefix: 'manacore',
+		currentAppId,
+	};
+}

packages/shared-splitscreen/src/utils/url-state.ts

@@ -0,0 +1,65 @@
+/**
+ * URL State Utilities
+ * Handle URL-based state persistence for split-screen.
+ */
+
+import type { UrlState } from '../types.js';
+
+/**
+ * Parse split-screen state from URL search params.
+ * Reads `?panel=todo&split=60` format.
+ */
+export function parseUrlState(searchParams: URLSearchParams): UrlState {
+	const panel = searchParams.get('panel') || undefined;
+	const splitStr = searchParams.get('split');
+	const split = splitStr ? parseInt(splitStr, 10) : undefined;
+
+	return {
+		panel,
+		split: split && !isNaN(split) ? split : undefined,
+	};
+}
+
+/**
+ * Update URL with split-screen state without page reload.
+ * Uses replaceState to avoid adding to browser history.
+ */
+export function updateUrlState(state: UrlState): void {
+	if (typeof window === 'undefined') return;
+
+	const url = new URL(window.location.href);
+
+	if (state.panel) {
+		url.searchParams.set('panel', state.panel);
+	} else {
+		url.searchParams.delete('panel');
+	}
+
+	if (state.split && state.split !== 50) {
+		url.searchParams.set('split', state.split.toString());
+	} else {
+		url.searchParams.delete('split');
+	}
+
+	window.history.replaceState({}, '', url.toString());
+}
+
+/**
+ * Clear split-screen state from URL.
+ */
+export function clearUrlState(): void {
+	if (typeof window === 'undefined') return;
+
+	const url = new URL(window.location.href);
+	url.searchParams.delete('panel');
+	url.searchParams.delete('split');
+	window.history.replaceState({}, '', url.toString());
+}
+
+/**
+ * Get current URL state.
+ */
+export function getCurrentUrlState(): UrlState {
+	if (typeof window === 'undefined') return {};
+	return parseUrlState(new URLSearchParams(window.location.search));
+}