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/error.html

@@ -0,0 +1,17 @@
+{% extends "base.html" %}
+
+{% block title %}Error{% endblock %}
+
+{% block content %}
+<div class="container mt-5">
+  <div class="row justify-content-center">
+    <div class="col-md-7">
+      <div class="alert alert-danger">
+        <h4 class="alert-heading">Something went wrong</h4>
+        <p class="mb-0"><code>{{ message }}</code></p>
+      </div>
+      <a href="{{ url_for('index') }}" class="btn btn-outline-secondary btn-sm">Back to home</a>
+    </div>
+  </div>
+</div>
+{% endblock %}