feat: improve admin login UI and sidebar branding

- Login page: split layout with gradient panel (desktop) + form,
  ProsApp logo mark, feature list, consistent blue brand colors
- Sidebar: replace plain text with logo mark + brand name

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

admin/src/components/sidebar.tsx

@@ -50,7 +50,19 @@ export default function Sidebar({ children }: { children: React.ReactNode }) {
         )}
       >
         <div className="flex h-14 items-center justify-between border-b px-3">
-          {!collapsed && <span className="font-semibold">ProsApp Admin</span>}
+          {!collapsed && (
+            <div className="flex items-center gap-2">
+              <div className="flex h-7 w-7 items-center justify-center rounded-lg bg-[#42A4EF]">
+                <span className="text-xs font-black text-white">P</span>
+              </div>
+              <span className="font-bold text-[#42A4EF]">ProsApp</span>
+            </div>
+          )}
+          {collapsed && (
+            <div className="flex h-7 w-7 items-center justify-center rounded-lg bg-[#42A4EF] mx-auto">
+              <span className="text-xs font-black text-white">P</span>
+            </div>
+          )}
           <Button variant="ghost" size="icon" onClick={() => setCollapsed(!collapsed)}>
             {collapsed ? <Menu size={18} /> : <ChevronLeft size={18} />}
           </Button>