fix: use config paths for static file mounts, redesign knowledge graph

- Fix generated images 404: use settings.generated_images_dir and
  settings.upload_dir instead of hardcoded relative paths in main.py
- Redesign knowledge graph with p5-style particle background, glow
  nodes, and curved gradient links
- Update course-content page border styling

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

web/src/app/(main)/course-content/page.tsx

@@ -150,7 +150,7 @@ export default function CourseContentPage() {
           <>
             {/* 知识图谱视图 */}
             {viewMode === 'graph' && (
-              <div className="w-full h-[calc(100vh-250px)] min-h-[600px] lg:min-h-[700px] rounded-lg border border-border bg-background overflow-hidden">
+              <div className="w-full h-[calc(100vh-250px)] min-h-[600px] lg:min-h-[700px] rounded-lg border border-white/10 overflow-hidden">
                 <KnowledgeGraph bookStructure={bookStructure} />
               </div>
             )}