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/router.go

@@ -69,6 +69,22 @@ func NewRouter(s *Server) chi.Router {
 					item.Delete("/", s.handleDeleteUser)
 				})
 			})
+
+			protected.Route("/files", func(files chi.Router) {
+				files.Get("/capabilities", s.handleCapabilities)
+				files.Get("/roots", s.handleListRoots)
+				files.Get("/", s.handleList)
+				files.Get("/info", s.handleInfo)
+				files.Post("/mkdir", s.handleMkdir)
+				files.Post("/rename", s.handleRename)
+				files.Post("/chmod", s.handleChmod)
+				files.Post("/chown", s.handleChown)
+				files.Delete("/", s.handleDelete)
+				files.Post("/upload", s.handleUpload)
+				files.Get("/download", s.handleDownload)
+				files.Get("/preview", s.handlePreview)
+				files.Get("/search", s.handleSearch)
+			})
 		})
 	})