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/js/advocate.js

@@ -833,6 +833,7 @@
     const score = s.reranker_score != null ? s.reranker_score : s.similarity;
     const originTagClass = s.source_origin === 'upload' ? 'dr-source-tag dr-source-tag--upload' : 'dr-source-tag';
     const originLabel = s.source_origin === 'upload' ? 'upload' : 'corpus';
+    const graphExpanded = s.graph_expanded === true;
     const link = s.deep_link || s.source_url;
     const titleHtml = link
       ? `<a href="${escapeHtml(link)}" target="_blank" rel="noopener" class="dr-source-title-link">${escapeHtml(s.title || 'Untitled')} <span class="dr-external-link" aria-hidden="true">↗</span></a>`
@@ -844,6 +845,7 @@
         ${s.section ? `<div class="dr-source-meta"><span class="dr-source-tag">${escapeHtml(s.section)}</span></div>` : ''}
         <div class="dr-source-meta">
           <span class="${originTagClass}">${originLabel}</span>
+          ${graphExpanded ? `<span class="dr-source-tag dr-source-tag--graph">via citation graph</span>` : ''}
           ${s.authority_label ? `<span class="dr-source-tag">${escapeHtml(s.authority_label)}</span>` : ''}
           <span class="dr-source-tag dr-source-tag--score">${escapeHtml(s.package_or_corpus || '—')}</span>
           ${(s.matched_sub_questions || []).map((q) => `<span class="dr-source-tag">${escapeHtml(q)}</span>`).join('')}