fix: improve note interactions and markdown LaTeX support

## Bug Fixes

### Note Card Actions
- Fix broken size change functionality (missing state declaration)
- Implement React 19 useOptimistic for instant UI feedback
- Add startTransition for non-blocking updates
- Ensure smooth animations without page refresh
- All note actions now work: pin, archive, color, size, checklist

### Markdown LaTeX Rendering
- Add remark-math and rehype-katex plugins
- Support inline equations with dollar sign syntax
- Support block equations with double dollar sign syntax
- Import KaTeX CSS for proper styling
- Equations now render correctly instead of showing raw LaTeX

## Technical Details

- Replace undefined currentNote references with optimistic state
- Add optimistic updates before server actions for instant feedback
- Use router.refresh() in transitions for smart cache invalidation
- Install remark-math, rehype-katex, and katex packages

## Testing

- Build passes successfully with no TypeScript errors
- Dev server hot-reloads changes correctly

keep-notes/components/user-nav.tsx

@@ -0,0 +1,75 @@
+'use client'
+
+import {
+  DropdownMenu,
+  DropdownMenuContent,
+  DropdownMenuGroup,
+  DropdownMenuItem,
+  DropdownMenuLabel,
+  DropdownMenuSeparator,
+  DropdownMenuTrigger,
+} from '@/components/ui/dropdown-menu'
+import { Button } from '@/components/ui/button'
+import { Avatar, AvatarFallback, AvatarImage } from '@/components/ui/avatar'
+import { useSession, signOut } from 'next-auth/react'
+import { useRouter } from 'next/navigation'
+import { LogOut, Settings, User, Shield } from 'lucide-react'
+
+export function UserNav({ user }: { user?: any }) {
+  const { data: session } = useSession()
+  const router = useRouter()
+
+  const currentUser = user || session?.user
+
+  if (!currentUser) return null
+
+  const userRole = (currentUser as any).role || currentUser.role
+  const userInitials = currentUser.name
+    ? currentUser.name.split(' ').map((n: string) => n[0]).join('').toUpperCase().substring(0, 2)
+    : 'U'
+
+  return (
+    <DropdownMenu>
+      <DropdownMenuTrigger asChild>
+        <Button variant="ghost" className="relative h-8 w-8 rounded-full">
+          <Avatar className="h-8 w-8">
+            <AvatarImage src={currentUser.image || ''} alt={currentUser.name || ''} />
+            <AvatarFallback>{userInitials}</AvatarFallback>
+          </Avatar>
+        </Button>
+      </DropdownMenuTrigger>
+      <DropdownMenuContent className="w-56" align="end" forceMount>
+        <DropdownMenuLabel className="font-normal">
+          <div className="flex flex-col space-y-1">
+            <p className="text-sm font-medium leading-none">{currentUser.name}</p>
+            <p className="text-xs leading-none text-muted-foreground">
+              {currentUser.email}
+            </p>
+          </div>
+        </DropdownMenuLabel>
+        <DropdownMenuSeparator />
+        <DropdownMenuGroup>
+          <DropdownMenuItem onClick={() => router.push('/settings/profile')}>
+            <User className="mr-2 h-4 w-4" />
+            <span>Profile</span>
+          </DropdownMenuItem>
+          {userRole === 'ADMIN' && (
+            <DropdownMenuItem onClick={() => router.push('/admin')}>
+              <Shield className="mr-2 h-4 w-4" />
+              <span>Admin Dashboard</span>
+            </DropdownMenuItem>
+          )}
+          <DropdownMenuItem onClick={() => router.push('/settings')}>
+            <Settings className="mr-2 h-4 w-4" />
+            <span>Diagnostics</span>
+          </DropdownMenuItem>
+        </DropdownMenuGroup>
+        <DropdownMenuSeparator />
+        <DropdownMenuItem onClick={() => signOut({ callbackUrl: '/login' })}>
+          <LogOut className="mr-2 h-4 w-4" />
+          <span>Log out</span>
+        </DropdownMenuItem>
+      </DropdownMenuContent>
+    </DropdownMenu>
+  )
+}