Redesign UI and fix several bugs

- Add navbar with login/logout, search form, and Find Link branding
- Clean up index page: search-only, examples behind ?debug=1
- Improve article page: remove debug clutter, named Wikipedia links, collapsible candidates
- Add SVG favicon (🔗 emoji)
- Fix diff CSS: compact layout, auto table layout to eliminate wide marker column gap
- Catch TokenRequestDenied in OAuth start and show error page
- Store username in session at login; clear bad session on API failure
- Raise NoMatch when diff is empty (edit already applied)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

templates/all_done.html

@@ -1,10 +1,11 @@
 {% extends "base.html" %}
 
-{% block title %}Index{% endblock %}
+{% block title %}All done{% endblock %}
 
 {% block content %}
-  <div class="container">
-    <h1>All done</h1>
-    <div><a href="{{ url_for('index') }}">back to index </a></div>
-  </div>
+<div class="container text-center mt-5">
+  <h1 class="mb-3">All done</h1>
+  <p class="text-muted mb-4">No more candidates found for this article.</p>
+  <a href="{{ url_for('index') }}" class="btn btn-primary">Search another article</a>
+</div>
 {% endblock %}