Add Citation Explorer tool and graph-expansion badges to Advocate results

- citations.php + assets/js/citations.js: new tool page for browsing the
  FalkorDB citation graph by title/ID, with autocomplete, action pills
  (cites/cited_by/implements/chain), hop-by-hop navigation, and exploration trail
- advocate.js: tag graph-expanded source cards with 'via citation graph' badge
- DeepResearchAgent: propagate _graph_expanded flag through normalizeCorpusChunk
  and top_sources serialization so it reaches the frontend
- tools.css: add .dr-source-tag--graph variant (green pill)
- i18n.php: register 'citations' tool in all 4 languages with CIT icon

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

assets/css/tools.css

@@ -2672,7 +2672,8 @@ p {
 }
 
 .dr-source-tag--upload { background: #fff0e8; color: #8a4524; }
-.dr-source-tag--score { background: #eef3fb; color: #314158; }
+.dr-source-tag--score  { background: #eef3fb; color: #314158; }
+.dr-source-tag--graph  { background: #f0fdf4; color: #166534; }
 
 .dr-source-excerpt {
   color: var(--muted);