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/app_files.go

@@ -2,12 +2,29 @@ package httpserver
 
 import (
 	"net/http"
+	"net/http/httputil"
+	"net/url"
 	"os"
 	"path/filepath"
 	"strings"
 )
 
 func (s *Server) appFileServer() http.Handler {
+	if s.config.Web.DevViteURL != "" {
+		viteURL, err := url.Parse(s.config.Web.DevViteURL)
+		if err == nil {
+			proxy := httputil.NewSingleHostReverseProxy(viteURL)
+			originalDirector := proxy.Director
+			proxy.Director = func(req *http.Request) {
+				originalDirector(req)
+				req.Host = viteURL.Host
+				// Add back the /app/ prefix that was stripped by http.StripPrefix
+				req.URL.Path = "/app" + req.URL.Path
+			}
+			return proxy
+		}
+	}
+
 	files := http.FileServer(http.Dir(s.config.Web.DistDir))
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		cleanPath := filepath.Clean(strings.TrimPrefix(r.URL.Path, "/"))