style: refine admin & me UI — card forms, back arrows, avatar in admin sidebar, Empty component

- Replace ← text with icon-[ri--arrow-left-line] in admin and me layouts
- Add avatar + admin shield badge to admin sidebar header
- Wrap all admin edit forms in Card (bg-card/50 border-primary/20) with styled inputs
- Fix sm:pl-6 → lg:pl-6 so extra left padding only applies when sidebar is visible
- Update security form submit button to gradient style matching profile
- Remove "View Public Profile" button from me/profile
- Use shadcn-svelte Empty component for recordings empty state
- Install empty component via shadcn-svelte

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/frontend/src/lib/components/ui/empty/empty-header.svelte

@@ -0,0 +1,20 @@
+<script lang="ts">
+	import { cn, type WithElementRef } from "$lib/utils.js";
+	import type { HTMLAttributes } from "svelte/elements";
+
+	let {
+		ref = $bindable(null),
+		class: className,
+		children,
+		...restProps
+	}: WithElementRef<HTMLAttributes<HTMLDivElement>> = $props();
+</script>
+
+<div
+	bind:this={ref}
+	data-slot="empty-header"
+	class={cn("flex max-w-sm flex-col items-center gap-2 text-center", className)}
+	{...restProps}
+>
+	{@render children?.()}
+</div>