feat: miller column browser, offline cache, search, breadcrumbs

- Dynamic miller column sizing with flexbox layout
- Root column (160px), middle slices (48px), active column (flex)
- Auto-scroll browser to rightmost column on navigation
- Offline indicator UI and IndexedDB cache integration
- /api/documents endpoint for client-side document caching
- Breadcrumb navigation in document content area
- FTS5 search with SQLite virtual table
- Client-side Mermaid and KaTeX rendering via CDN
- Deep sample content (advanced-topics/raft-deep-dive)
- Border removal (only search button keeps radius)
- Full viewport layout with proper borders between sidebar/content

apps/server/internal/httpserver/middleware.go

@@ -39,7 +39,7 @@ func (s *Server) requestLogger(next http.Handler) http.Handler {
 
 func (s *Server) securityHeaders(next http.Handler) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		w.Header().Set("Content-Security-Policy", "default-src 'self'; base-uri 'self'; frame-ancestors 'none'; img-src 'self' data:; style-src 'self' 'unsafe-inline'; script-src 'self'; connect-src 'self' ws: wss:; object-src 'none'")
+		w.Header().Set("Content-Security-Policy", "default-src 'self'; base-uri 'self'; frame-ancestors 'none'; img-src 'self' data:; style-src 'self' 'unsafe-inline' https://cdn.jsdelivr.net; script-src 'self' https://cdn.jsdelivr.net; font-src 'self' https://cdn.jsdelivr.net; connect-src 'self' ws: wss:; object-src 'none'")
 		w.Header().Set("Referrer-Policy", "strict-origin-when-cross-origin")
 		w.Header().Set("X-Content-Type-Options", "nosniff")
 		w.Header().Set("X-Frame-Options", "DENY")