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/app/(main)/admin/user-list.tsx

@@ -0,0 +1,82 @@
+'use client'
+
+import { useState } from 'react'
+import { Button } from '@/components/ui/button'
+import { deleteUser, updateUserRole } from '@/app/actions/admin'
+import { toast } from 'sonner'
+import { Trash2, Shield, ShieldOff } from 'lucide-react'
+import { format } from 'date-fns'
+
+export function UserList({ initialUsers }: { initialUsers: any[] }) {
+  
+  // Optimistic update could be implemented here, but standard is fine for admin
+  const handleDelete = async (id: string) => {
+    if (!confirm('Are you sure? This action cannot be undone.')) return
+    try {
+      await deleteUser(id)
+      toast.success('User deleted')
+    } catch (e) {
+      toast.error('Failed to delete')
+    }
+  }
+
+  const handleRoleToggle = async (user: any) => {
+    const newRole = user.role === 'ADMIN' ? 'USER' : 'ADMIN'
+    try {
+      await updateUserRole(user.id, newRole)
+      toast.success(`User role updated to ${newRole}`)
+    } catch (e) {
+      toast.error('Failed to update role')
+    }
+  }
+
+  return (
+    <div className="w-full overflow-auto">
+      <table className="w-full caption-bottom text-sm text-left">
+        <thead className="[&_tr]:border-b">
+          <tr className="border-b transition-colors hover:bg-muted/50 data-[state=selected]:bg-muted">
+            <th className="h-12 px-4 align-middle font-medium text-muted-foreground">Name</th>
+            <th className="h-12 px-4 align-middle font-medium text-muted-foreground">Email</th>
+            <th className="h-12 px-4 align-middle font-medium text-muted-foreground">Role</th>
+            <th className="h-12 px-4 align-middle font-medium text-muted-foreground">Created At</th>
+            <th className="h-12 px-4 align-middle font-medium text-muted-foreground text-right">Actions</th>
+          </tr>
+        </thead>
+        <tbody className="[&_tr:last-child]:border-0">
+          {initialUsers.map((user) => (
+            <tr key={user.id} className="border-b transition-colors hover:bg-muted/50 data-[state=selected]:bg-muted">
+              <td className="p-4 align-middle font-medium">{user.name || 'N/A'}</td>
+              <td className="p-4 align-middle">{user.email}</td>
+              <td className="p-4 align-middle">
+                <span className={`inline-flex items-center rounded-full border px-2.5 py-0.5 text-xs font-semibold transition-colors focus:outline-none focus:ring-2 focus:ring-ring focus:ring-offset-2 ${user.role === 'ADMIN' ? 'border-transparent bg-primary text-primary-foreground hover:bg-primary/80' : 'border-transparent bg-secondary text-secondary-foreground hover:bg-secondary/80'}`}>
+                  {user.role}
+                </span>
+              </td>
+              <td className="p-4 align-middle">{format(new Date(user.createdAt), 'PP')}</td>
+              <td className="p-4 align-middle text-right">
+                <div className="flex justify-end gap-2">
+                  <Button
+                    variant="ghost"
+                    size="sm"
+                    onClick={() => handleRoleToggle(user)}
+                    title={user.role === 'ADMIN' ? "Demote to User" : "Promote to Admin"}
+                  >
+                    {user.role === 'ADMIN' ? <ShieldOff className="h-4 w-4" /> : <Shield className="h-4 w-4" />}
+                  </Button>
+                  <Button
+                    variant="ghost"
+                    size="sm"
+                    onClick={() => handleDelete(user.id)}
+                    className="text-red-600 hover:text-red-900"
+                  >
+                    <Trash2 className="h-4 w-4" />
+                  </Button>
+                </div>
+              </td>
+            </tr>
+          ))}
+        </tbody>
+      </table>
+    </div>
+  )
+}