feat(todo/web): add page maximize/minimize/close controls and default todo page

- Show "To Do" page by default on app load
- Add fullscreen (maximize) button to TodoPage with centered content
- Consistent button order across all states: Minimize → Fullscreen → Close
- Add maximize/restore/close buttons to minimized page tabs
- Use ArrowLineUp icon for restore in tabs to differentiate from minimize
- Larger tap targets for tab action buttons (24px)
- Centered empty state when no pages are open with "Seite hinzufügen" button
- Wire maximize through MinimizedPagesContext for tab-to-fullscreen flow

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/shared-ui/src/bottom-stack/index.ts

@@ -0,0 +1,4 @@
+export { default as BottomStack } from './BottomStack.svelte';
+export { default as MinimizedTabs } from './MinimizedTabs.svelte';
+export { default as NotificationBar } from './NotificationBar.svelte';
+export type { MinimizedPage, MinimizedTabsCallbacks, BottomNotification } from './types';