todos: add xlogin shim — pod-authenticated writes via one script tag

Drops in xlogin (Nostr NIP-07/NIP-98 + Solid OIDC/DPoP) as a single
script. Adds the purple Login button bottom-right; on login, exposes
window.xlogin.authFetch which LOSOS's todo-pane and schema-pane already
prefer over plain fetch ((window.xlogin && window.xlogin.authFetch) ||
fetch). Result: when someone forks this app and points it at a real
pod URL (?uri=https://my.pod/today.jsonld), edits sync back via DPoP-
or NIP-98-authenticated PUTs. No code changes needed.

The data is currently inline so writes don't actually persist — but
the auth wiring is now complete so a fork-with-pod-URL Just Works.

Refs: https://github.com/melvincarvalho/xlogin

Author: melvincarvalho

todos/index.html

@@ -67,6 +67,10 @@
 }
 </script>
 
+<!-- Login (xlogin: Nostr + Solid). Adds the purple Login button bottom-right;
+     LOSOS panes pick up window.xlogin.authFetch for authenticated writes. -->
+<script src="https://unpkg.com/xlogin"></script>
+
 <!-- Panes — todo-pane handles the Tracker, schema-view + source for individual Vtodos -->
 <script type="module" data-pane src="https://losos.org/panes/todo-pane.js"></script>
 <script type="module" data-pane src="https://losos.org/panes/schema-pane.js"></script>