Move auth template to page

lib/nulla_web/components/templates/page/sign_in.html.heex

@@ -0,0 +1,9 @@
+<main class="flex min-h-screen items-center justify-center">
+  <form class="flex flex-col gap-2" action="/auth/sign_in" method="post">
+    <label for="email">E-mail address *</label>
+    <input id="email" name="email" type="email" maxlength="50" required />
+    <label for="password">Password *</label>
+    <input id="password" name="password" type="password" minlength="8" maxlength="50" required />
+    <button class="text-white bg-black py-2 border" type="submit">Log in</button>
+  </form>
+</main>