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.

web/templates/partials/settings_nav.html

@@ -3,6 +3,9 @@
   <a href="/settings" class="list-group-item list-group-item-action{{if eq . "general"}} active{{end}}">
     <i class="bi bi-sliders me-2"></i>General
   </a>
+  <a href="/settings/statuses" class="list-group-item list-group-item-action{{if eq . "statuses"}} active{{end}}">
+    <i class="bi bi-tag me-2"></i>Statuses
+  </a>
   <a href="/settings/item-types" class="list-group-item list-group-item-action{{if eq . "itemtypes"}} active{{end}}">
     <i class="bi bi-tags me-2"></i>Item Types
   </a>