fix: Explorer fetch failure (missing language param) + add Explore nav button

- Fix 422 error on /explore by adding required `language` query param to wiki_cache fetch
- Add "Explore" button to wiki page toolbar for navigation to Visual Explorer
- Integrate structured diagram data prompt into wiki page generation so new wikis produce explorable data
- Preserve language/type params in back-navigation links

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

Author: 2 people

package-lock.json

@@ -53,6 +53,7 @@ export default function ExplorePage() {
   const owner = params.owner as string;
   const repo = params.repo as string;
   const repoType = (searchParams.get('type') || 'github') as 'github' | 'gitlab' | 'bitbucket';
+  const language = searchParams.get('language') || 'en';
 
   // State from URL or defaults
   const [selectedView, setSelectedView] = useState<ExplorerView>(
@@ -80,6 +81,7 @@ export default function ExplorePage() {
           owner,
           repo,
           repo_type: repoType,
+          language,
         });
         const response = await fetch(`/api/wiki_cache?${cacheParams.toString()}`);
         if (!response.ok) {
@@ -114,7 +116,7 @@ export default function ExplorePage() {
     }
 
     fetchWikiData();
-  }, [owner, repo, repoType]);
+  }, [owner, repo, repoType, language]);
 
   // Merge all diagram data for the selected view into one DiagramData
   const mergedDiagram = useMemo<DiagramData | null>(() => {
@@ -176,8 +178,11 @@ export default function ExplorePage() {
     setSelectedDepth(depth);
   }, []);
 
-  // Build back link
-  const backHref = `/${owner}/${repo}${searchParams.get('type') ? `?type=${repoType}` : ''}`;
+  // Build back link (preserve type and language params)
+  const backParams = new URLSearchParams();
+  if (searchParams.get('type')) backParams.set('type', repoType);
+  if (searchParams.get('language')) backParams.set('language', language);
+  const backHref = `/${owner}/${repo}${backParams.toString() ? `?${backParams.toString()}` : ''}`;
 
   // No diagram data state
   const hasNoDiagramData = !isLoading && !error && allDiagrams.length === 0;

src/app/[owner]/[repo]/explore/page.tsx

@@ -641,6 +641,24 @@ Based ONLY on the content of the \`[RELEVANT_SOURCE_FILES]\`:
          - Use autonumber directive to add sequence numbers to messages
          - NEVER use flowchart-style labels like A--|label|-->B. Always use a colon for labels: A->>B: My Label
 
+    *   **Structured Diagram Data:** When you generate a Mermaid diagram, you MUST ALSO produce a structured JSON block IMMEDIATELY BEFORE the corresponding Mermaid code fence. Wrap the JSON in HTML comment markers exactly like this:
+        \`\`\`
+        <!-- DIAGRAM_DATA_START -->
+        {
+          "nodes": [
+            {"id": "A", "label": "Frontend App", "technology": "React", "files": ["src/App.tsx"], "description": "Main SPA entry point", "depth": 0},
+            {"id": "B", "label": "API Server", "technology": "Express", "files": ["server/index.js"], "description": "REST API backend", "depth": 0}
+          ],
+          "edges": [
+            {"source": "A", "target": "B", "label": "HTTP requests", "type": "api_call"}
+          ],
+          "mermaidSource": "graph TD\\n    A[Frontend App] --> B[API Server]",
+          "diagramType": "flowchart"
+        }
+        <!-- DIAGRAM_DATA_END -->
+        \`\`\`
+        Rules: "nodes[].id" must match the Mermaid node IDs. "edges[].type" must be "dependency", "data_flow", or "api_call". "diagramType" must be "flowchart", "sequence", "class", or "er". "mermaidSource" must contain the exact Mermaid source. If a diagram has no meaningful structured metadata, you may omit the JSON block.
+
 4.  **Tables:**
     *   Use Markdown tables to summarize information such as:
         *   Key features or components and their descriptions.
@@ -2176,6 +2194,19 @@ IMPORTANT:
                     <span className="hidden sm:inline">Graph</span>
                   </button>
                 )}
+                {/* Visual Explorer button */}
+                {!isLoading && wikiStructure && (
+                  <Link
+                    href={`/${owner}/${repo}/explore${repoType !== 'github' ? `?type=${repoType}` : ''}${language !== 'en' ? `${repoType !== 'github' ? '&' : '?'}language=${language}` : ''}`}
+                    className="inline-flex items-center gap-2 rounded-md border border-input bg-background px-3 py-1.5 text-sm text-muted-foreground shadow-sm transition-colors hover:bg-accent hover:text-accent-foreground"
+                    title="Interactive Visual Explorer"
+                  >
+                    <svg className="h-3.5 w-3.5" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={2}>
+                      <path strokeLinecap="round" strokeLinejoin="round" d="M3.75 6A2.25 2.25 0 016 3.75h2.25A2.25 2.25 0 0110.5 6v2.25a2.25 2.25 0 01-2.25 2.25H6a2.25 2.25 0 01-2.25-2.25V6zM3.75 15.75A2.25 2.25 0 016 13.5h2.25a2.25 2.25 0 012.25 2.25V18a2.25 2.25 0 01-2.25 2.25H6A2.25 2.25 0 013.75 18v-2.25zM13.5 6a2.25 2.25 0 012.25-2.25H18A2.25 2.25 0 0120.25 6v2.25A2.25 2.25 0 0118 10.5h-2.25a2.25 2.25 0 01-2.25-2.25V6zM13.5 15.75a2.25 2.25 0 012.25-2.25H18a2.25 2.25 0 012.25 2.25V18A2.25 2.25 0 0118 20.25h-2.25A2.25 2.25 0 0113.5 18v-2.25z" />
+                    </svg>
+                    <span className="hidden sm:inline">Explore</span>
+                  </Link>
+                )}
                 {/* Reading mode toggle button */}
                 {!isLoading && wikiStructure && (
                   <button