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/breadcrumb.html

@@ -5,7 +5,7 @@
     {{$last := sub (len .) 1}}
     {{range $i, $b := .}}
       {{if eq $i $last}}
-      <li class="breadcrumb-item active" aria-current="page">{{$b.Name}}</li>
+      <li class="breadcrumb-item active" aria-current="page" data-context="folder-header" data-id="{{$b.ID}}">{{$b.Name}}</li>
       {{else}}
       <li class="breadcrumb-item"><a href="#" class="breadcrumb-link" data-type="folder" data-id="{{$b.ID}}">{{$b.Name}}</a></li>
       {{end}}