feat(shared-ui): unify ImmersiveModeToggle across Calendar, Contacts, and Todo apps

- Move ImmersiveModeToggle component from Calendar to shared-ui package
- Add immersiveModeEnabled setting to Contacts and Todo settings stores
- Update all three app layouts with F-key shortcut and conditional UI rendering
- Consistent glass-pill styling on hover for toggle button

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/shared-ui/src/components/ImmersiveModeToggle.svelte

@@ -0,0 +1,113 @@
+<script lang="ts">
+	import { ChevronDown, ChevronUp } from 'lucide-svelte';
+
+	interface Props {
+		/** Whether immersive mode is currently enabled */
+		isImmersive: boolean;
+		/** Callback to toggle immersive mode */
+		onToggle: () => void;
+		/** Whether to show the toggle (e.g., only on main page) */
+		visible?: boolean;
+	}
+
+	let { isImmersive, onToggle, visible = true }: Props = $props();
+</script>
+
+{#if visible}
+	<button
+		class="immersive-toggle"
+		class:immersive={isImmersive}
+		onclick={onToggle}
+		title={isImmersive ? 'UI anzeigen (F)' : 'UI verstecken (F)'}
+	>
+		{#if isImmersive}
+			<ChevronUp size={20} />
+		{:else}
+			<ChevronDown size={20} />
+		{/if}
+	</button>
+{/if}
+
+<style>
+	.immersive-toggle {
+		position: fixed;
+		bottom: 0;
+		left: 50%;
+		transform: translateX(-50%);
+		z-index: 9999;
+		display: flex;
+		align-items: center;
+		justify-content: center;
+		width: 80px;
+		height: 24px;
+		border-radius: 8px 8px 0 0;
+		border: none;
+		background: transparent;
+		color: hsl(var(--color-muted-foreground));
+		cursor: pointer;
+		pointer-events: auto;
+		transition:
+			opacity 300ms ease,
+			background 150ms ease,
+			color 150ms ease;
+	}
+
+	.immersive-toggle:hover {
+		background: rgba(255, 255, 255, 0.85);
+		backdrop-filter: blur(12px);
+		-webkit-backdrop-filter: blur(12px);
+		border: 1px solid rgba(0, 0, 0, 0.1);
+		color: #374151;
+		box-shadow:
+			0 4px 6px -1px rgba(0, 0, 0, 0.1),
+			0 2px 4px -2px rgba(0, 0, 0, 0.1);
+	}
+
+	.immersive-toggle:active {
+		background: rgba(255, 255, 255, 0.95);
+		backdrop-filter: blur(12px);
+		-webkit-backdrop-filter: blur(12px);
+	}
+
+	/* Dark mode hover */
+	:global(.dark) .immersive-toggle:hover {
+		background: rgba(255, 255, 255, 0.12);
+		border: 1px solid rgba(255, 255, 255, 0.15);
+		color: #f3f4f6;
+	}
+
+	:global(.dark) .immersive-toggle:active {
+		background: rgba(255, 255, 255, 0.2);
+	}
+
+	/* Immersive mode: even more subtle, full opacity on hover */
+	.immersive-toggle.immersive {
+		opacity: 0.2;
+		color: hsl(var(--color-muted-foreground));
+	}
+
+	.immersive-toggle.immersive:hover {
+		opacity: 1;
+		background: rgba(255, 255, 255, 0.85);
+		backdrop-filter: blur(12px);
+		-webkit-backdrop-filter: blur(12px);
+		border: 1px solid rgba(0, 0, 0, 0.1);
+		color: #374151;
+		box-shadow:
+			0 4px 6px -1px rgba(0, 0, 0, 0.1),
+			0 2px 4px -2px rgba(0, 0, 0, 0.1);
+	}
+
+	:global(.dark) .immersive-toggle.immersive:hover {
+		background: rgba(255, 255, 255, 0.12);
+		border: 1px solid rgba(255, 255, 255, 0.15);
+		color: #f3f4f6;
+	}
+
+	/* Mobile adjustments */
+	@media (max-width: 640px) {
+		.immersive-toggle {
+			bottom: env(safe-area-inset-bottom);
+		}
+	}
+</style>