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/package.json

@@ -0,0 +1,39 @@
+{
+	"name": "@manacore/shared-splitscreen",
+	"version": "0.1.0",
+	"private": true,
+	"type": "module",
+	"svelte": "./src/index.ts",
+	"main": "./src/index.ts",
+	"types": "./src/index.ts",
+	"exports": {
+		".": {
+			"svelte": "./src/index.ts",
+			"types": "./src/index.ts",
+			"default": "./src/index.ts"
+		},
+		"./store": {
+			"svelte": "./src/stores/split-panel.svelte.ts",
+			"default": "./src/stores/split-panel.svelte.ts"
+		},
+		"./types": {
+			"types": "./src/types.ts",
+			"default": "./src/types.ts"
+		},
+		"./utils": {
+			"default": "./src/utils/index.ts"
+		}
+	},
+	"scripts": {
+		"lint": "eslint .",
+		"check": "svelte-check --tsconfig ./tsconfig.json"
+	},
+	"peerDependencies": {
+		"svelte": "^5.0.0"
+	},
+	"devDependencies": {
+		"svelte": "^5.0.0",
+		"svelte-check": "^4.0.0",
+		"typescript": "^5.0.0"
+	}
+}