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/router.go

@@ -40,6 +40,7 @@ func (a *App) Router() http.Handler {
 	mux.HandleFunc("GET /app/search", a.requireLogin(a.handleSearch))
 
 	// Folder mutations (JSON in/out)
+	mux.HandleFunc("GET /app/folders/options", a.requireAdmin(a.handleFolderOptions))
 	mux.HandleFunc("GET /app/folders/{id}", a.requireAdmin(a.handleFolderGet))
 	mux.HandleFunc("POST /app/folders", a.requireAdmin(a.handleFolderCreate))
 	mux.HandleFunc("PATCH /app/folders/{id}", a.requireAdmin(a.handleFolderUpdate))