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_session_html/confirm.html.heex

@@ -0,0 +1,57 @@
+<div class="mx-auto max-w-sm">
+  <div class="text-center">
+    <.header>Welcome {@user.email}</.header>
+  </div>
+
+  <.form
+    :if={!@user.confirmed_at}
+    for={@form}
+    id="confirmation_form"
+    action={~p"/users/log-in?_action=confirmed"}
+    phx-mounted={JS.focus_first()}
+  >
+    <input type="hidden" name={@form[:token].name} value={@form[:token].value} />
+    <.button
+      name={@form[:remember_me].name}
+      value="true"
+      phx-disable-with="Confirming..."
+      class="btn btn-primary w-full"
+    >
+      Confirm and stay logged in
+    </.button>
+    <.button phx-disable-with="Confirming..." class="btn btn-primary btn-soft w-full mt-2">
+      Confirm and log in only this time
+    </.button>
+  </.form>
+
+  <.form
+    :if={@user.confirmed_at}
+    for={@form}
+    id="login_form"
+    action={~p"/users/log-in"}
+    phx-mounted={JS.focus_first()}
+  >
+    <input type="hidden" name={@form[:token].name} value={@form[:token].value} />
+    <%= if @current_scope do %>
+      <.button variant="primary" phx-disable-with="Logging in..." class="btn btn-primary w-full">
+        Log in
+      </.button>
+    <% else %>
+      <.button
+        name={@form[:remember_me].name}
+        value="true"
+        phx-disable-with="Logging in..."
+        class="btn btn-primary w-full"
+      >
+        Keep me logged in on this device
+      </.button>
+      <.button phx-disable-with="Logging in..." class="btn btn-primary btn-soft w-full mt-2">
+        Log me in only this time
+      </.button>
+    <% end %>
+  </.form>
+
+  <p :if={!@user.confirmed_at} class="alert alert-outline mt-8">
+    Tip: If you prefer passwords, you can enable them in the user settings.
+  </p>
+</div>