Blank root panel, asset form as a modal, and a dedicated Statuses page

- Main panel is now blank at the top level ("Select a folder or asset
  from the tree") instead of showing a dashboard - browsing happens in
  the tree, the panel only shows something once you click into it.
  Removed the now-dead DashboardStats/Stats() machinery it depended on.
- Add/Edit Asset no longer takes over the main panel; it opens in
  #assetModal instead; asset_form.html was trimmed down to just the form
  (no more breadcrumb/panel wrapper) since the modal chrome provides the
  title and footer buttons now. The dynamic item-type-fields and submit
  listeners moved from #main-panel's delegation to the modal element's.
- Statuses split out of the General settings page into its own
  /settings/statuses page and sub-nav entry, alongside General, Item
  Types, and Users.

internal/web/router.go

@@ -64,6 +64,7 @@ func (a *App) Router() http.Handler {
 	// sub-navigation, rather than as separate top-level nav items.
 	mux.HandleFunc("GET /settings", a.requireAdmin(a.handleSettingsPage))
 	mux.HandleFunc("POST /settings/general", a.requireAdmin(a.handleSettingsGeneral))
+	mux.HandleFunc("GET /settings/statuses", a.requireAdmin(a.handleSettingsStatusesPage))
 	mux.HandleFunc("POST /settings/statuses", a.requireAdmin(a.handleStatusCreate))
 	mux.HandleFunc("POST /settings/statuses/{id}", a.requireAdmin(a.handleStatusUpdate))
 	mux.HandleFunc("POST /settings/statuses/{id}/delete", a.requireAdmin(a.handleStatusDelete))