feat: file manager with browse, search, upload, chmod/chown

- Full file browser page at /files with lazy-load, breadcrumbs, drag&drop
- FileBrowserModal component for path selection from Samba/NFS forms
- PathField component replaces bare inputs in share/export forms
- Backend: /api/files/* routes with List, Mkdir, Rename, Delete, Chmod, Chown, Upload, Download, Preview, Search
- Reuses NASCTL_ALLOWED_ROOTS for path validation
- NASCTL_UPLOAD_MAX_BYTES (100MB) and NASCTL_PREVIEW_MAX_BYTES (256KB) env vars
- Capabilities endpoint returns chmod/chown availability (requires root)
- Version bump: 0.3.2 -> 0.4.0

internal/web/auth.go

@@ -190,29 +190,41 @@ func (s *Server) RequireAuth(next http.Handler) http.Handler {
 }
 
 type Server struct {
-	DB           *db.DB
-	Engine       *engine.Engine
-	AllowedRoots []string
-	Auth         *AuthService
-	SMBConfPath  string
-	ExportsPath  string
+	DB             *db.DB
+	Engine         *engine.Engine
+	AllowedRoots   []string
+	Auth           *AuthService
+	SMBConfPath    string
+	ExportsPath    string
+	UploadMaxBytes  int64
+	PreviewMaxBytes int64
 }
 
 type Options struct {
-	AllowedRoots []string
-	Auth         *AuthService
-	SMBConfPath  string
-	ExportsPath  string
+	AllowedRoots    []string
+	Auth            *AuthService
+	SMBConfPath     string
+	ExportsPath     string
+	UploadMaxBytes  int64
+	PreviewMaxBytes int64
 }
 
 func NewServer(database *db.DB, eng *engine.Engine, opts Options) *Server {
+	if opts.UploadMaxBytes == 0 {
+		opts.UploadMaxBytes = 100 << 20 // 100 MB
+	}
+	if opts.PreviewMaxBytes == 0 {
+		opts.PreviewMaxBytes = 256 << 10 // 256 KB
+	}
 	return &Server{
-		DB:           database,
-		Engine:       eng,
-		AllowedRoots: opts.AllowedRoots,
-		Auth:         opts.Auth,
-		SMBConfPath:  opts.SMBConfPath,
-		ExportsPath:  opts.ExportsPath,
+		DB:             database,
+		Engine:         eng,
+		AllowedRoots:   opts.AllowedRoots,
+		Auth:           opts.Auth,
+		SMBConfPath:    opts.SMBConfPath,
+		ExportsPath:    opts.ExportsPath,
+		UploadMaxBytes:  opts.UploadMaxBytes,
+		PreviewMaxBytes: opts.PreviewMaxBytes,
 	}
 }