feat: wire reply intelligence to frontend — timeline filtering, thread reconstruction, visibility badges

- Filter isContext items and private/direct posts from main timeline, new post count, and unread count
- Post detail: query local replies from ap_timeline before remote fetch, deduplicate, sort chronologically
- Add visibility badge (unlisted/private/direct) on item cards next to timestamp

Confab-Link: http://localhost:8080/sessions/af5f8b45-6b8d-442d-8f25-78c326190709

views/partials/ap-item-card.njk

@@ -65,6 +65,9 @@
         </time>
         {% if item.updated %}<span class="ap-card__edited" title="{{ item.updated | date('PPp') }}">✏️</span>{% endif %}
       </a>
+      {% if item.visibility and item.visibility != "public" %}
+        <span class="ap-card__visibility ap-card__visibility--{{ item.visibility }}" title="{% if item.visibility == 'unlisted' %}{{ __('activitypub.reader.compose.visibilityUnlisted') }}{% elif item.visibility == 'private' %}{{ __('activitypub.reader.compose.visibilityFollowers') }}{% elif item.visibility == 'direct' %}DM{% endif %}">{% if item.visibility == "unlisted" %}🔓{% elif item.visibility == "private" %}🔒{% elif item.visibility == "direct" %}✉️{% endif %}</span>
+      {% endif %}
     {% endif %}
   </header>