Update tag styling to match desktop app

Change tag display from transparent background with colored text
to solid colored background with white text to match the desktop app:
- Background: Solid tag color (was transparent with 20% opacity)
- Text: White (was same color as tag)
- Maintains existing rounded rectangle shape from CSS

Tags now appear as colored pills with white text, matching the
visual style of the desktop application.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

templates/tasks/_task_item.html

@@ -28,7 +28,7 @@
             <span class="task-meta-separator">·</span>
             {% endif %}
             {% for tag in task.tags.all %}
-            <span class="task-group-label" style="background-color: {{ tag.color }}20; color: {{ tag.color }}">
+            <span class="task-group-label" style="background-color: {{ tag.color }}; color: white;">
                 {{ tag.name }}
             </span>
             {% endfor %}