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/i18n/locales/en.ts

@@ -96,8 +96,8 @@ export default {
       security: "Security",
       recordings: "Recordings",
       analytics: "Analytics",
-      back_to_site: "← Back to site",
-      back_mobile: "← Back",
+      back_to_site: "Back to site",
+      back_mobile: "Back",
     },
     analytics: {
       title: "Analytics",
@@ -922,8 +922,8 @@ export default {
   },
   admin: {
     nav: {
-      back_to_site: "← Back to site",
-      back_mobile: "← Back",
+      back_to_site: "Back to site",
+      back_mobile: "Back",
       title: "Admin",
       users: "Users",
       videos: "Videos",