Add category search, license handling, and message page improvements

Add /category route to find Wikipedia articles without images in a
category using the MediaWiki API. Filter out non-content images (UI
icons, logos) when checking articles.

Show image license on message page with alternate message for non-free
CC licenses (NC/ND) explaining Wikipedia's restrictions. For photos
with free licenses, show upload options linking to UploadWizard instead
of a message form.

Add Flickr CC 4.0 license codes, user profile links, previous message
detection from sent mail index, and back-navigation between category,
search results, and message pages.

Closes #3

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

templates/category.html

@@ -0,0 +1,67 @@
+{% extends "base.html" %}
+
+{% block title %}Category Search - Flickr mail{% endblock %}
+
+{% block style %}
+<style>
+  .article-link:visited { color: #6f42c1; }
+</style>
+{% endblock %}
+
+{% block content %}
+  <div class="container">
+    <div class="row">
+    <h1>Find articles needing images</h1>
+    <p class="text-muted">Enter a Wikipedia category to find articles without images</p>
+
+    <form action="{{ url_for('category_search') }}" method="get">
+      <div class="mb-3">
+        <label for="cat" class="form-label">Wikipedia category name or URL:</label>
+        <input type="text" class="form-control" id="cat" name="cat" value="{{ cat }}"
+               placeholder="e.g., Living people or https://en.wikipedia.org/wiki/Category:Living_people" required>
+      </div>
+      <input type="submit" class="btn btn-primary" value="Search">
+      <a href="{{ url_for('start') }}" class="btn btn-outline-secondary ms-2">Back to main</a>
+    </form>
+
+    {% if error %}
+    <div class="alert alert-danger mt-3">{{ error }}</div>
+    {% endif %}
+
+    {% if category and articles is defined %}
+    <div class="mt-4">
+      <h5>Articles without images in <a href="https://en.wikipedia.org/wiki/{{ category | replace(' ', '_') }}" target="_blank">{{ category_name }}</a></h5>
+
+      {% if articles %}
+      <p class="text-muted small">Found {{ articles | length }} article(s) without images{% if continue_token %} (more available){% endif %}</p>
+
+      <div class="list-group">
+        {% for article in articles %}
+        <div class="list-group-item d-flex justify-content-between align-items-center">
+          <a href="{{ url_for('start', enwp=article.title, cat=cat) }}" class="text-decoration-none article-link">{{ article.title }}</a>
+          <a href="{{ article.wikipedia_url }}" target="_blank" class="badge bg-secondary text-decoration-none">Wikipedia</a>
+        </div>
+        {% endfor %}
+      </div>
+
+      {% if continue_token %}
+      <p class="text-muted small mt-3">Note: Only showing first batch of results. More articles may be available in this category.</p>
+      {% endif %}
+
+      {% else %}
+      <div class="alert alert-success mt-3">
+        All articles in this category have images!
+      </div>
+      {% endif %}
+    </div>
+    {% endif %}
+
+    <div class="mt-4">
+      <p class="text-muted small">
+        <a href="{{ url_for('start') }}">Back to Flickr mail home</a>
+      </p>
+    </div>
+
+    </div>
+  </div>
+{% endblock %}