refactor: presentation becomes http-axum adapter, absorbs templates

crates/adapters/http-axum/templates/register.html

@@ -0,0 +1,25 @@
+{% extends "base.html" %}
+{% block content %}
+<h1>Register</h1>
+{% if let Some(err) = error %}
+<p class="error">{{ err }}</p>
+{% endif %}
+<form method="POST" action="/register">
+  <label>
+    Username<br>
+    <small>2–30 chars: letters, digits, underscores, hyphens. This becomes your ActivityPub handle.</small><br>
+    <input type="text" name="username" required autocomplete="username"
+           pattern="[a-z0-9_\-]{2,30}" title="2–30 lowercase letters, digits, underscores or hyphens">
+  </label>
+  <label>
+    Email<br>
+    <input type="email" name="email" required autocomplete="email">
+  </label>
+  <label>
+    Password<br>
+    <input type="password" name="password" required autocomplete="new-password">
+  </label>
+  <input type="hidden" name="_csrf" value="{{ ctx.csrf_token }}">
+  <button type="submit">Register</button>
+</form>
+{% endblock %}