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/login-form.tsx

@@ -16,7 +16,7 @@ function LoginButton() {
   );
 }
 
-export function LoginForm() {
+export function LoginForm({ allowRegister = true }: { allowRegister?: boolean }) {
   const [errorMessage, dispatch] = useActionState(authenticate, undefined);
 
   return (
@@ -64,6 +64,14 @@ export function LoginForm() {
             </div>
           </div>
         </div>
+        <div className="flex items-center justify-end mt-2">
+          <Link
+            href="/forgot-password"
+            className="text-xs text-gray-500 hover:text-gray-900 underline"
+          >
+            Forgot password?
+          </Link>
+        </div>
         <LoginButton />
         <div
           className="flex h-8 items-end space-x-1"
@@ -74,12 +82,14 @@ export function LoginForm() {
             <p className="text-sm text-red-500">{errorMessage}</p>
           )}
         </div>
-        <div className="mt-4 text-center text-sm">
-          Don't have an account?{' '}
-          <Link href="/register" className="underline">
-            Register
-          </Link>
-        </div>
+        {allowRegister && (
+          <div className="mt-4 text-center text-sm">
+            Don't have an account?{' '}
+            <Link href="/register" className="underline">
+              Register
+            </Link>
+          </div>
+        )}
       </div>
     </form>
   );