Use HX-Target, not HX-Boost for Layout render

HX-Boost is not always sent in all AJAX requests that htmx sends, only
those initiated by an element with hx-boost enabled. It was not showing
up on requests following an HX-Redirect response.

After reading the docs more, I realized HX-Target was what I wanted. If
I can see that the request is targeting `#main-content` then I know to
only return HTML inside that element. Simple.

src/partials/login_form.rs

@@ -16,7 +16,7 @@ pub fn login_form(props: LoginFormProps) -> Markup {
         general_error,
     } = props;
     html! {
-        form hx-post="/login" hx-swap="outerHTML" class="auth-form-grid" {
+        form action="/login" method="post" class="auth-form-grid" {
             label for="email" { "Email" }
             input type="email" name="email" id="email" placeholder="Email" value=(email.unwrap_or_default()) required;
             @if let Some(email_error) = email_error {