Replace on-screen action buttons with right-click context menus

The tree and main panels now behave like a native file manager: right
click (or the keyboard Menu key) on a folder, asset, breadcrumb crumb, or
empty tree/panel background brings up the relevant actions instead of a
row of always-visible buttons.

- New generic showContextMenu() in app.js with per-target-kind menu
  builders (folder/asset/tree background/panel background), gated by
  admin role the same way the old buttons were.
- Removed: the "..." tree-row action button, and the New Folder/Add
  Asset/Rename/Delete/Edit/Move/Duplicate/Import/Export buttons from the
  inventory toolbar and every panel. Expand All/Collapse All shrank to two
  icon-only buttons next to the search box; everything else creation- or
  mutation-related is right-click only.
- Replaced the per-asset embedded "Move" modal with one shared
  #moveAssetModal populated via a new GET /app/folders/options endpoint,
  since move can now be triggered from any asset row, not just its own
  detail view.
- Breadcrumb crumbs (including the current folder) and folder/asset table
  rows are now valid right-click targets too, not just tree rows.

web/templates/partials/tree_children.html

@@ -12,11 +12,6 @@
       {{end}}
       <i class="bi {{iconOr .Icon "bi-folder-fill"}} tree-icon text-warning-emphasis"></i>
       <span class="tree-label">{{.Name}}</span>
-      <span class="tree-actions admin-only">
-        <button type="button" class="btn-tree-action" data-action="menu" data-id="{{.ID}}" data-type="folder" title="Actions">
-          <i class="bi bi-three-dots"></i>
-        </button>
-      </span>
     </div>
     <ul class="tree-children list-unstyled{{if not .Expanded}} d-none{{end}}" data-expanded="{{.Expanded}}" data-loaded="{{.Expanded}}">
       {{if .Expanded}}{{template "tree_children" .Children}}{{end}}