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/config/config.go

@@ -31,7 +31,8 @@ type ContentConfig struct {
 }
 
 type WebConfig struct {
-	DistDir string `json:"distDir"`
+	DistDir    string `json:"distDir"`
+	DevViteURL string `json:"devViteUrl"`
 }
 
 func Default() Config {
@@ -47,7 +48,8 @@ func Default() Config {
 			StoreDir:  filepath.Join("..", "..", "data", "files"),
 		},
 		Web: WebConfig{
-			DistDir: filepath.Join("..", "web", "dist"),
+			DistDir:    filepath.Join("..", "web", "dist"),
+			DevViteURL: os.Getenv("MD_HUB_DEV_VITE_URL"),
 		},
 		LogLevel: "INFO",
 	}
@@ -69,6 +71,7 @@ func Load() (Config, error) {
 	overrideString(&cfg.Content.SourceDir, "MD_HUB_CONTENT_SOURCE_DIR")
 	overrideString(&cfg.Content.StoreDir, "MD_HUB_CONTENT_STORE_DIR")
 	overrideString(&cfg.Web.DistDir, "MD_HUB_WEB_DIST_DIR")
+	overrideString(&cfg.Web.DevViteURL, "MD_HUB_DEV_VITE_URL")
 	overrideString(&cfg.LogLevel, "MD_HUB_LOG_LEVEL")
 
 	if cfg.Server.Addr == "" {