feat(#72): implement interactive graph visualization component

- Create KnowledgeGraphViewer component with @xyflow/react
- Implement three layout types: force-directed, hierarchical (ELK), circular
- Add node sizing based on connection count (40px-120px range)
- Apply PDA-friendly status colors (green=published, blue=draft, gray=archived)
- Highlight orphan nodes with distinct color
- Add interactive features: zoom, pan, click-to-navigate
- Implement filters: status, tags, show/hide orphans
- Add statistics display and legend panel
- Create comprehensive test suite (16 tests, all passing)
- Add fetchKnowledgeGraph API function
- Create /knowledge/graph page
- Performance tested with 500+ nodes
- All quality gates passed (tests, typecheck, lint)

Refs #72

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/web/src/lib/api/knowledge.ts

@@ -318,6 +318,59 @@ export async function fetchEntryGraph(
   return apiGet(`/api/knowledge/entries/${slug}/graph?${params.toString()}`);
 }
 
+/**
+ * Fetch full knowledge graph
+ */
+export async function fetchKnowledgeGraph(filters?: {
+  tags?: string[];
+  status?: string;
+  limit?: number;
+}): Promise<{
+  nodes: {
+    id: string;
+    slug: string;
+    title: string;
+    summary: string | null;
+    status?: string;
+    tags: {
+      id: string;
+      name: string;
+      slug: string;
+      color: string | null;
+    }[];
+    depth: number;
+    isOrphan?: boolean;
+  }[];
+  edges: {
+    id: string;
+    sourceId: string;
+    targetId: string;
+    linkText: string;
+  }[];
+  stats: {
+    totalNodes: number;
+    totalEdges: number;
+    orphanCount: number;
+  };
+}> {
+  const params = new URLSearchParams();
+  if (filters?.tags && filters.tags.length > 0) {
+    filters.tags.forEach((tag) => {
+      params.append("tags", tag);
+    });
+  }
+  if (filters?.status) {
+    params.append("status", filters.status);
+  }
+  if (filters?.limit !== undefined) {
+    params.append("limit", filters.limit.toString());
+  }
+
+  const queryString = params.toString();
+  const endpoint = queryString ? `/api/knowledge/graph?${queryString}` : "/api/knowledge/graph";
+  return apiGet(endpoint);
+}
+
 /**
  * Mock entries for development (until backend endpoints are ready)
  */