refactor(templates): _tag_cards.html and reader.html use tag_id

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/templates/reader.html

@@ -8,7 +8,7 @@
   <!-- Reader Header -->
   <div class="reader-header">
     <div class="reader-header-left">
-      <a href="{{ url_for('main.gallery', tag=tag.name) }}" class="reader-back-btn" title="Back to gallery">
+      <a href="{{ url_for('main.gallery', tag_id=tag.id) }}" class="reader-back-btn" title="Back to gallery">
         <span class="back-arrow">&larr;</span>
       </a>
       <h1 class="reader-title">{{ series_name }}</h1>
@@ -63,7 +63,7 @@
         {% else %}
           <div class="reader-empty">
             <p>No pages in this series yet.</p>
-            <a href="{{ url_for('main.gallery', tag=tag.name) }}" class="btn primary-btn">
+            <a href="{{ url_for('main.gallery', tag_id=tag.id) }}" class="btn primary-btn">
               Go to gallery to add pages
             </a>
           </div>