fix: load SignalIdModal on dashboard pages; clear status on zero matches

- Added signal-id-modal.js script to base_dashboard.html (nav button was silent on dashboard pages)
- Added else branch to clear "Searching…" status when zero matches returned

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

templates/layout/base_dashboard.html

@@ -184,6 +184,7 @@
 
 {% block scripts %}
 {{ super() }}
+<script src="{{ url_for('static', filename='js/components/signal-id-modal.js') }}{% if version %}?v={{ version }}{% endif %}"></script>
 <script>
     // Dashboard-specific scripts
     (function() {