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/create-user-dialog.tsx

@@ -0,0 +1,78 @@
+'use client'
+
+import { useState } from 'react'
+import { Button } from '@/components/ui/button'
+import {
+  Dialog,
+  DialogContent,
+  DialogDescription,
+  DialogHeader,
+  DialogTitle,
+  DialogTrigger,
+  DialogFooter,
+} from '@/components/ui/dialog'
+import { Input } from '@/components/ui/input'
+import { Plus } from 'lucide-react'
+import { createUser } from '@/app/actions/admin'
+import { toast } from 'sonner'
+
+export function CreateUserDialog() {
+  const [open, setOpen] = useState(false)
+  
+  return (
+    <Dialog open={open} onOpenChange={setOpen}>
+      <DialogTrigger asChild>
+        <Button>
+          <Plus className="mr-2 h-4 w-4" /> Add User
+        </Button>
+      </DialogTrigger>
+      <DialogContent className="sm:max-w-[425px]">
+        <DialogHeader>
+          <DialogTitle>Create User</DialogTitle>
+          <DialogDescription>
+            Add a new user to the system. They will need to change their password upon first login if you implement that policy.
+          </DialogDescription>
+        </DialogHeader>
+        <form
+          action={async (formData) => {
+            const result = await createUser(formData)
+            if (result?.error) {
+              toast.error('Failed to create user')
+            } else {
+              toast.success('User created successfully')
+              setOpen(false)
+            }
+          }}
+          className="grid gap-4 py-4"
+        >
+          <div className="grid gap-2">
+            <label htmlFor="name">Name</label>
+            <Input id="name" name="name" required />
+          </div>
+          <div className="grid gap-2">
+            <label htmlFor="email">Email</label>
+            <Input id="email" name="email" type="email" required />
+          </div>
+          <div className="grid gap-2">
+            <label htmlFor="password">Password</label>
+            <Input id="password" name="password" type="password" required minLength={6} />
+          </div>
+          <div className="grid gap-2">
+            <label htmlFor="role">Role</label>
+            <select
+              id="role"
+              name="role"
+              className="flex h-9 w-full rounded-md border border-input bg-transparent px-3 py-1 text-sm shadow-sm transition-colors file:border-0 file:bg-transparent file:text-sm file:font-medium placeholder:text-muted-foreground focus-visible:outline-none focus-visible:ring-1 focus-visible:ring-ring disabled:cursor-not-allowed disabled:opacity-50"
+            >
+              <option value="USER">User</option>
+              <option value="ADMIN">Admin</option>
+            </select>
+          </div>
+          <DialogFooter>
+            <Button type="submit">Create User</Button>
+          </DialogFooter>
+        </form>
+      </DialogContent>
+    </Dialog>
+  )
+}