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/go.mod

@@ -11,6 +11,8 @@ require (
 
 require (
 	github.com/antlr4-go/antlr/v4 v4.13.0 // indirect
+	github.com/fsnotify/fsnotify v1.9.0 // indirect
 	github.com/libsql/sqlite-antlr4-parser v0.0.0-20240327125255-dbf53b6cbf06 // indirect
 	golang.org/x/exp v0.0.0-20230515195305-f3d0a9c9a5cc // indirect
+	golang.org/x/sys v0.13.0 // indirect
 )