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/index.html

@@ -1,25 +1,44 @@
 {% extends "base.html" %}
 
-{% block title %}Index{% endblock %}
+{% block title %}Find Link{% endblock %}
 
 {% block content %}
-  <div class="container">
-    <h1>Index</h1>
-    <form>
-      <input name="q">
-      <input type="submit" value="search">
-    </form>
-
-    <div>Username: {{ g.user }}</div>
-
-    <table class="table w-auto">
-      {% for item in examples %}
-      <tr>
-        <td><a href="{{ article_url(item.title) }}">{{ item.title }}</a></td>
-        <td>{{ item.total }}</td>
-        <td>{{ "{:.1%}".format(item.with_links / item.total) }}</td>
-      </tr>
-      {% endfor %}
-    </table>
+<div class="container">
+  <div class="row justify-content-center mt-5">
+    <div class="col-md-6 text-center">
+      <h1 class="mb-2">Find Link</h1>
+      <p class="text-muted mb-4">Find unlinked mentions of a Wikipedia article and add the links.</p>
+      <form class="d-flex gap-2 justify-content-center" action="{{ url_for('index') }}">
+        <input class="form-control" name="q" placeholder="Article title…" style="max-width:360px" autofocus>
+        <button class="btn btn-primary" type="submit">Search</button>
+      </form>
+    </div>
   </div>
+
+  {% if debug %}
+  <div class="row mt-5">
+    <div class="col">
+      <h2 class="h6 text-muted text-uppercase mb-3">Examples</h2>
+      <table class="table table-sm table-hover w-auto">
+        <thead class="table-light">
+          <tr>
+            <th>Article</th>
+            <th class="text-end">Total</th>
+            <th class="text-end">% linked</th>
+          </tr>
+        </thead>
+        <tbody>
+        {% for item in examples %}
+        <tr>
+          <td><a href="{{ article_url(item.title) }}">{{ item.title }}</a></td>
+          <td class="text-end text-muted">{{ item.total }}</td>
+          <td class="text-end text-muted">{{ "{:.0%}".format(item.with_links / item.total) }}</td>
+        </tr>
+        {% endfor %}
+        </tbody>
+      </table>
+    </div>
+  </div>
+  {% endif %}
+</div>
 {% endblock %}