Add phx.gen.auth authentication scaffolding

- LiveView-based email/password auth via mix phx.gen.auth
- Auth links removed from public navigation (direct URL access only)
- Accounts context with User schema and token management

app/lib/firehose_web/controllers/user_registration_html/new.html.heex

@@ -0,0 +1,30 @@
+<div class="mx-auto max-w-sm">
+  <div class="text-center">
+    <.header>
+      Register for an account
+      <:subtitle>
+        Already registered?
+        <.link navigate={~p"/users/log-in"} class="font-semibold text-brand hover:underline">
+          Log in
+        </.link>
+        to your account now.
+      </:subtitle>
+    </.header>
+  </div>
+
+  <.form :let={f} for={@changeset} action={~p"/users/register"}>
+    <.input
+      field={f[:email]}
+      type="email"
+      label="Email"
+      autocomplete="username"
+      spellcheck="false"
+      required
+      phx-mounted={JS.focus()}
+    />
+
+    <.button phx-disable-with="Creating account..." class="btn btn-primary w-full">
+      Create an account
+    </.button>
+  </.form>
+</div>