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/routes/me/+layout.svelte

@@ -44,9 +44,10 @@
       <div class="flex items-center gap-1 overflow-x-auto py-2 scrollbar-none">
         <a
           href="/"
-          class="shrink-0 text-xs text-muted-foreground hover:text-foreground transition-colors px-2"
+          class="shrink-0 flex items-center gap-1 text-xs text-muted-foreground hover:text-foreground transition-colors px-2"
         >
-          {$_("me.nav.back_mobile")}
+          <span class="icon-[ri--arrow-left-line] h-4 w-4"></span>
+          <span class="hidden sm:inline">{$_("me.nav.back_mobile")}</span>
         </a>
         {#each navLinks as link (link.href)}
           <a
@@ -69,7 +70,8 @@
       <!-- Sidebar (desktop only) -->
       <aside class="hidden lg:flex w-56 shrink-0 flex-col border-r border-border/40">
         <div class="px-4 py-5 border-b border-border/40">
-          <a href="/" class="text-xs text-muted-foreground hover:text-foreground transition-colors">
+          <a href="/" class="flex items-center gap-1 text-xs text-muted-foreground hover:text-foreground transition-colors">
+            <span class="icon-[ri--arrow-left-line] h-3.5 w-3.5"></span>
             {$_("me.nav.back_to_site")}
           </a>
           <div class="mt-3 flex items-center gap-3">