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>

includes/DeepResearchAgent.php

@@ -374,6 +374,7 @@ final class DbnDeepResearchAgent
                     'source_url'     => $s['source_url'] ?? null,
                     'source_origin'  => $s['source_origin'] ?? 'corpus',
                     'authority_label'=> $s['authority_label'] ?? null,
+                    'graph_expanded' => $s['graph_expanded'] ?? false,
                     'excerpt'        => $s['excerpt'] ?? '',
                 ], $topSources),
             ];
@@ -733,6 +734,7 @@ PROMPT;
             'similarity'        => $similarity,
             'reranker_score'    => $rerankerScore,
             'document_id'       => isset($chunk['document_id']) ? (int)$chunk['document_id'] : null,
+            'graph_expanded'    => !empty($chunk['_graph_expanded']),
             'source_origin'     => 'corpus',
             'authority_type'    => $chunk['authority_type'] ?? null,
             'jurisdiction'      => $chunk['jurisdiction'] ?? null,