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_folders.go

@@ -8,6 +8,17 @@ import (
 	"inventree/internal/models"
 )
 
+// handleFolderOptions returns every folder as a flat, indented list for
+// generic "move to..." pickers (e.g. the asset move context-menu action).
+func (a *App) handleFolderOptions(w http.ResponseWriter, r *http.Request) {
+	opts, err := a.folderOptions(r.Context())
+	if err != nil {
+		jsonErr(w, http.StatusInternalServerError, err)
+		return
+	}
+	jsonOK(w, opts)
+}
+
 func (a *App) handleFolderGet(w http.ResponseWriter, r *http.Request) {
 	id, err := pathID(r, "id")
 	if err != nil {