refactor(admin): phase 7 — delete old tab components and page.server.ts

Remove UsersTab, GroupsTab, TagsTab, SystemTab and their specs; delete
the monolithic +page.server.ts with shared load + 6 form actions (all
now handled by dedicated sub-route servers under users/, groups/, tags/).
Add delete action and confirmation button to user edit panel.
Fix test to query the edit form by id rather than the first form in DOM.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/admin/users/[id]/+page.server.ts

@@ -1,4 +1,4 @@
-import { error, fail } from '@sveltejs/kit';
+import { error, fail, redirect } from '@sveltejs/kit';
 import type { PageServerLoad, Actions } from './$types';
 import { createApiClient } from '$lib/api.server';
 import { getErrorMessage } from '$lib/errors';
@@ -63,5 +63,19 @@ export const actions: Actions = {
 		}
 
 		return { success: true };
+	},
+
+	delete: async ({ params, fetch }) => {
+		const api = createApiClient(fetch);
+		const result = await api.DELETE('/api/users/{id}', {
+			params: { path: { id: params.id } }
+		});
+
+		if (!result.response.ok) {
+			const code = (result.error as unknown as { code?: string })?.code;
+			return fail(result.response.status, { error: getErrorMessage(code) });
+		}
+
+		throw redirect(303, '/admin/users');
 	}
 };