Implement user settings and dark mode toggle in layout component

- Added functionality to toggle dark mode based on user preferences, with state management and API integration for saving settings.
- Introduced a user avatar display with initials fallback and a dropdown menu for logout and dark mode options.
- Enhanced layout structure to conditionally render the header based on user authentication status.

src/lib/components/ui/avatar/avatar-fallback.svelte

@@ -0,0 +1,17 @@
+<script lang="ts">
+	import { Avatar as AvatarPrimitive } from "bits-ui";
+	import { cn } from "$lib/utils.js";
+
+	let {
+		ref = $bindable(null),
+		class: className,
+		...restProps
+	}: AvatarPrimitive.FallbackProps = $props();
+</script>
+
+<AvatarPrimitive.Fallback
+	bind:ref
+	data-slot="avatar-fallback"
+	class={cn("bg-muted flex size-full items-center justify-center rounded-full", className)}
+	{...restProps}
+/>

src/lib/components/ui/avatar/avatar-image.svelte

@@ -0,0 +1,17 @@
+<script lang="ts">
+	import { Avatar as AvatarPrimitive } from "bits-ui";
+	import { cn } from "$lib/utils.js";
+
+	let {
+		ref = $bindable(null),
+		class: className,
+		...restProps
+	}: AvatarPrimitive.ImageProps = $props();
+</script>
+
+<AvatarPrimitive.Image
+	bind:ref
+	data-slot="avatar-image"
+	class={cn("aspect-square size-full", className)}
+	{...restProps}
+/>

src/lib/components/ui/avatar/avatar.svelte

@@ -0,0 +1,19 @@
+<script lang="ts">
+	import { Avatar as AvatarPrimitive } from "bits-ui";
+	import { cn } from "$lib/utils.js";
+
+	let {
+		ref = $bindable(null),
+		loadingStatus = $bindable("loading"),
+		class: className,
+		...restProps
+	}: AvatarPrimitive.RootProps = $props();
+</script>
+
+<AvatarPrimitive.Root
+	bind:ref
+	bind:loadingStatus
+	data-slot="avatar"
+	class={cn("relative flex size-8 shrink-0 overflow-hidden rounded-full", className)}
+	{...restProps}
+/>

src/lib/components/ui/avatar/index.ts

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+import Root from "./avatar.svelte";
+import Image from "./avatar-image.svelte";
+import Fallback from "./avatar-fallback.svelte";
+
+export {
+	Root,
+	Image,
+	Fallback,
+	//
+	Root as Avatar,
+	Image as AvatarImage,
+	Fallback as AvatarFallback,
+};

src/lib/components/ui/switch/index.ts

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+import Root from "./switch.svelte";
+
+export {
+	Root,
+	//
+	Root as Switch,
+};

src/lib/components/ui/switch/switch.svelte

@@ -0,0 +1,29 @@
+<script lang="ts">
+	import { Switch as SwitchPrimitive } from "bits-ui";
+	import { cn, type WithoutChildrenOrChild } from "$lib/utils.js";
+
+	let {
+		ref = $bindable(null),
+		class: className,
+		checked = $bindable(false),
+		...restProps
+	}: WithoutChildrenOrChild<SwitchPrimitive.RootProps> = $props();
+</script>
+
+<SwitchPrimitive.Root
+	bind:ref
+	bind:checked
+	data-slot="switch"
+	class={cn(
+		"data-[state=checked]:bg-primary data-[state=unchecked]:bg-input focus-visible:border-ring focus-visible:ring-ring/50 dark:data-[state=unchecked]:bg-input/80 peer inline-flex h-[1.15rem] w-8 shrink-0 items-center rounded-full border border-transparent shadow-xs transition-all outline-none focus-visible:ring-[3px] disabled:cursor-not-allowed disabled:opacity-50",
+		className
+	)}
+	{...restProps}
+>
+	<SwitchPrimitive.Thumb
+		data-slot="switch-thumb"
+		class={cn(
+			"bg-background dark:data-[state=unchecked]:bg-foreground dark:data-[state=checked]:bg-primary-foreground pointer-events-none block size-4 rounded-full ring-0 transition-transform data-[state=checked]:translate-x-[calc(100%-2px)] data-[state=unchecked]:translate-x-0"
+		)}
+	/>
+</SwitchPrimitive.Root>