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.

internal/web/handlers_inventory.go

@@ -221,9 +221,7 @@ func (a *App) handlePanelFolder(w http.ResponseWriter, r *http.Request) {
 
 // AssetPanelData backs the main panel when an asset is selected (view mode).
 type AssetPanelData struct {
-	Asset         *models.Asset
-	CanEdit       bool
-	FolderOptions []FolderOption
+	Asset *models.Asset
 }
 
 func (a *App) handlePanelAsset(w http.ResponseWriter, r *http.Request) {
@@ -238,12 +236,7 @@ func (a *App) handlePanelAsset(w http.ResponseWriter, r *http.Request) {
 		writeErr(w, http.StatusNotFound, err.Error())
 		return
 	}
-	opts, err := a.folderOptions(ctx)
-	if err != nil {
-		writeErr(w, http.StatusInternalServerError, err.Error())
-		return
-	}
-	a.RenderFragment(w, http.StatusOK, "panel_asset", AssetPanelData{Asset: asset, CanEdit: currentUser(r).IsAdmin(), FolderOptions: opts})
+	a.RenderFragment(w, http.StatusOK, "panel_asset", AssetPanelData{Asset: asset})
 }
 
 // AssetFormData backs both the "new asset" and "edit asset" forms.