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/header/header.svelte

@@ -38,7 +38,7 @@
     isMobileMenuOpen = false;
   }
 
-  function isActiveLink(link: { name: string; href: string }) {
+  function isActiveLink(link: { name?: string; href: string }) {
     return (
       (page.url.pathname === "/" && link === navLinks[0]) ||
       (page.url.pathname.startsWith(link.href) && link !== navLinks[0])
@@ -80,20 +80,6 @@
       {#if authStatus.authenticated}
         <div class="w-full flex items-center justify-end">
           <div class="flex items-center gap-2 rounded-full bg-muted/30 p-1">
-            <Button
-              variant="link"
-              size="icon"
-              class={`flex h-9 w-9 rounded-full p-0 relative text-foreground/80 group ${isActiveLink({ href: "/me" }) ? "text-foreground" : "hover:text-foreground"}`}
-              href="/me"
-              title={$_("header.dashboard")}
-            >
-              <span class="icon-[ri--dashboard-2-line] h-4 w-4"></span>
-              <span
-                class={`absolute -bottom-1 left-0 w-0 h-0.5 bg-gradient-to-r from-primary to-accent transition-all duration-300 ${isActiveLink({ href: "/me" }) ? "w-full" : "group-hover:w-full"}`}
-              ></span>
-              <span class="sr-only">{$_("header.dashboard")}</span>
-            </Button>
-
             <Button
               variant="link"
               size="icon"