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.svelte

@@ -16,6 +16,25 @@ const selectedGroupIds = $derived(data.editUser.groups?.map((g: { id: string })
 		<h2 class="flex-1 font-sans text-sm font-bold text-ink">
 			{m.admin_user_edit_heading({ username: data.editUser.username })}
 		</h2>
+		<form
+			method="POST"
+			action="?/delete"
+			use:enhance={({ cancel }) => {
+				if (!confirm(m.admin_user_delete_confirm({ username: data.editUser.username }))) {
+					cancel();
+				}
+				return async ({ update }) => {
+					await update();
+				};
+			}}
+		>
+			<button
+				type="submit"
+				class="rounded-sm border border-red-200 bg-red-50 px-3 py-1 font-sans text-xs font-bold tracking-widest text-red-700 uppercase transition-colors hover:bg-red-100 dark:border-red-900 dark:bg-red-950/30 dark:text-red-400"
+			>
+				{m.btn_delete()}…
+			</button>
+		</form>
 	</div>
 
 	<!-- Scrollable body -->