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/static/css/app.css

@@ -129,25 +129,31 @@ ul.tree-children {
   flex-shrink: 0;
 }
 
-.tree-actions {
-  flex-shrink: 0;
-}
-
-.btn-tree-action {
-  border: none;
-  background: none;
-  color: #6c757d;
-  padding: 0 0.25rem;
-}
-
-.btn-tree-action:hover {
-  color: #212529;
-}
-
 .tree-node.dragging {
   opacity: 0.4;
 }
 
+/* Generic right-click context menu (tree + panels feel like a native app:
+   most actions live here instead of on-screen buttons). */
+.ctx-menu {
+  position: fixed;
+  z-index: 1080;
+  min-width: 190px;
+  display: none;
+}
+
+.ctx-menu .dropdown-item i {
+  width: 1.1rem;
+  display: inline-block;
+  text-align: center;
+  margin-right: 0.15rem;
+}
+
+[data-context="asset-header"],
+[data-context="folder-header"] {
+  cursor: default;
+}
+
 .stat-card .card-body {
   text-align: center;
 }