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/panel_asset.html

@@ -1,26 +1,16 @@
 {{define "panel_asset"}}
 <div class="panel-content p-3" data-asset-id="{{.Asset.ID}}">
-  <div class="d-flex align-items-start mb-2">
-    <nav aria-label="breadcrumb">
-      <ol class="breadcrumb mb-0">
-        <li class="breadcrumb-item"><a href="#" class="breadcrumb-link" data-type="root">Inventory</a></li>
-        {{range .Asset.FolderPath}}
-        <li class="breadcrumb-item"><a href="#" class="breadcrumb-link" data-type="folder" data-id="{{.ID}}">{{.Name}}</a></li>
-        {{end}}
-        <li class="breadcrumb-item active" aria-current="page">{{.Asset.Name}}</li>
-      </ol>
-    </nav>
-    {{if .CanEdit}}
-    <div class="ms-auto d-flex gap-2">
-      <button class="btn btn-sm btn-outline-secondary" data-action="edit-asset" data-id="{{.Asset.ID}}"><i class="bi bi-pencil"></i> Edit</button>
-      <button class="btn btn-sm btn-outline-secondary" data-action="move-asset" data-bs-toggle="modal" data-bs-target="#moveAssetModal-{{.Asset.ID}}"><i class="bi bi-arrows-move"></i> Move</button>
-      <button class="btn btn-sm btn-outline-secondary" data-action="duplicate-asset" data-id="{{.Asset.ID}}"><i class="bi bi-files"></i> Duplicate</button>
-      <button class="btn btn-sm btn-outline-danger" data-action="delete-asset" data-id="{{.Asset.ID}}" data-name="{{.Asset.Name}}"><i class="bi bi-trash"></i> Delete</button>
-    </div>
-    {{end}}
-  </div>
+  <nav aria-label="breadcrumb">
+    <ol class="breadcrumb mb-0">
+      <li class="breadcrumb-item"><a href="#" class="breadcrumb-link" data-type="root">Inventory</a></li>
+      {{range .Asset.FolderPath}}
+      <li class="breadcrumb-item"><a href="#" class="breadcrumb-link" data-type="folder" data-id="{{.ID}}">{{.Name}}</a></li>
+      {{end}}
+      <li class="breadcrumb-item active" aria-current="page">{{.Asset.Name}}</li>
+    </ol>
+  </nav>
 
-  <div class="d-flex align-items-center gap-2 mb-3">
+  <div class="d-flex align-items-center gap-2 mb-3 mt-2" data-context="asset-header" data-id="{{.Asset.ID}}" title="Right-click for actions">
     <i class="bi {{iconOr .Asset.ItemTypeIcon "bi-box-seam"}} fs-3 text-body-secondary"></i>
     <div>
       <h4 class="mb-0">{{.Asset.Name}}</h4>
@@ -75,30 +65,4 @@
     </div>
   </div>
 </div>
-
-{{if .CanEdit}}
-<div class="modal fade" id="moveAssetModal-{{.Asset.ID}}" tabindex="-1">
-  <div class="modal-dialog">
-    <div class="modal-content">
-      <div class="modal-header">
-        <h5 class="modal-title">Move "{{.Asset.Name}}"</h5>
-        <button type="button" class="btn-close" data-bs-dismiss="modal"></button>
-      </div>
-      <div class="modal-body">
-        <label class="form-label">Destination Folder</label>
-        <select class="form-select" id="moveAssetDestination-{{.Asset.ID}}">
-          <option value="">Inventory (top level)</option>
-          {{range .FolderOptions}}
-          <option value="{{.ID}}">{{range seq .Depth}}&nbsp;&nbsp;&nbsp;{{end}}{{.Path}}</option>
-          {{end}}
-        </select>
-      </div>
-      <div class="modal-footer">
-        <button type="button" class="btn btn-outline-secondary" data-bs-dismiss="modal">Cancel</button>
-        <button type="button" class="btn btn-primary" data-action="confirm-move-asset" data-id="{{.Asset.ID}}" data-select="moveAssetDestination-{{.Asset.ID}}">Move</button>
-      </div>
-    </div>
-  </div>
-</div>
-{{end}}
 {{end}}