Add Slug Parameter to Action Loader for Programmatic Identification (#5490)

* feat: add slug parameter to action-loader for programmatic identification

Add optional `slug` parameter to ActionLoaderHandle for easier identification via code or CSS. Update all loader.show() calls across the codebase to include descriptive slugs ('app-init', 'chat-rename', 'chat-delete', 'bulk-delete', 'chat-load', 'image-generation', 'legacy-loader'). Add data attributes (data-slug, data-loader-id, data-blocking) to toast content div. Expose slug via getter and make id private with getter.

* Apply suggestions from code review

Fix slug jsdoc wording

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* fix: Add identifier to second loader in img gen

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Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Cohee <18619528+Cohee1207@users.noreply.github.com>

public/scripts/bookmarks.js

@@ -700,6 +700,7 @@ export function initBookmarks() {
         }
 
         const loaderHandle = loader.show({
+            slug: 'chat-load',
             title: t`Chat History`,
             message: t`Loading chat…`,
             toastMode: loader.ToastMode.STATIC,