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/app/watcher.go

@@ -0,0 +1,101 @@
+package app
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/fsnotify/fsnotify"
+)
+
+type FileWatcher struct {
+	watcher  *fsnotify.Watcher
+	rootDir  string
+	onChange func(path string)
+	logger   *slog.Logger
+}
+
+func NewFileWatcher(rootDir string, logger *slog.Logger) (*FileWatcher, error) {
+	watcher, err := fsnotify.NewWatcher()
+	if err != nil {
+		return nil, fmt.Errorf("create fsnotify watcher: %w", err)
+	}
+
+	fw := &FileWatcher{
+		watcher: watcher,
+		rootDir: rootDir,
+		logger:  logger,
+	}
+
+	if err := fw.addWatches(); err != nil {
+		watcher.Close()
+		return nil, err
+	}
+
+	return fw, nil
+}
+
+func (fw *FileWatcher) addWatches() error {
+	return filepath.Walk(fw.rootDir, func(path string, info os.FileInfo, err error) error {
+		if err != nil {
+			return nil
+		}
+		if info.IsDir() {
+			if err := fw.watcher.Add(path); err != nil {
+				fw.logger.Warn("watch directory", "path", path, "error", err)
+			}
+		}
+		return nil
+	})
+}
+
+func (fw *FileWatcher) SetOnChange(fn func(path string)) {
+	fw.onChange = fn
+}
+
+func (fw *FileWatcher) Run(ctx context.Context) {
+	defer fw.watcher.Close()
+
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case event, ok := <-fw.watcher.Events:
+			if !ok {
+				return
+			}
+			if fw.shouldProcess(event) {
+				if event.Op&fsnotify.Create == fsnotify.Create {
+					if info, err := os.Stat(event.Name); err == nil && info.IsDir() {
+						fw.watcher.Add(event.Name)
+					}
+				}
+				if fw.onChange != nil {
+					fw.onChange(event.Name)
+				}
+			}
+		case err, ok := <-fw.watcher.Errors:
+			if !ok {
+				return
+			}
+			fw.logger.Warn("fsnotify error", "error", err)
+		}
+	}
+}
+
+func (fw *FileWatcher) shouldProcess(event fsnotify.Event) bool {
+	if event.Op&(fsnotify.Write|fsnotify.Create|fsnotify.Remove|fsnotify.Rename) == 0 {
+		return false
+	}
+	name := filepath.Base(event.Name)
+	if strings.HasPrefix(name, ".") || strings.HasSuffix(name, "~") {
+		return false
+	}
+	if filepath.Ext(event.Name) == ".md" || event.Op&fsnotify.Remove != 0 {
+		return true
+	}
+	return false
+}