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/docs/service.go

@@ -200,6 +200,10 @@ func (s *Service) syncFile(ctx context.Context, path string) (*DocumentChange, e
 		return nil, err
 	}
 
+	if err := s.repo.UpdateSearchContent(ctx, relative, string(content)); err != nil {
+		s.logger.Warn("index search content", "path", relative, "error", err)
+	}
+
 	s.logger.Debug("synced document", "path", relative, "hash", record.Hash)
 	return &DocumentChange{
 		Path:         relative,