Fix remaining 'Find Link' references and improve error messages

- Rename index page title and heading to 'Missing Link'
- Give a clear message for 429 rate limit errors
- Include response body snippet in other unexpected API errors
- Log warning when falling back to unauthenticated session

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

templates/index.html

@@ -1,12 +1,12 @@
 {% extends "base.html" %}
 
-{% block title %}Find Link{% endblock %}
+{% block title %}Missing Link{% endblock %}
 
 {% block content %}
 <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>
+      <h1 class="mb-2">Missing 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>