feat(sidebar): manage-collective link as last nav entry with spacer

The DesktopSidebar previously had no entry point for /collective/manage,
making member-management and the new common-items / tags subsections
unreachable from the chrome on desktop. The MobileDrawer already linked
it from inside the collective switcher block; desktop didn't.

- Adds a `<div aria-hidden>` divider with `my-2 border-t` to separate
  the section nav (lists / tasks / notes / search) from the manage
  entry.
- Adds a `Users`-iconed link to `/collective/manage` as the last nav
  item, with the same active-state styling as the other entries.
- Not gated by `$enabledSections` (membership management is orthogonal
  to feature toggles — same call as the Admin entry in the footer).
- Reuses the existing `m.manage_title()` Paraglide string; no new
  strings.

Tests: tests/e2e/sidebar-manage-link.test.ts adds SBM-01 (link visible
and navigates) + SBM-02 (divider present in <nav>).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

apps/web/src/lib/components/layout/DesktopSidebar.svelte

@@ -9,7 +9,7 @@
 	import CreateCollectiveModal from '$lib/components/CreateCollectiveModal.svelte';
 	import OfflineChip from '$lib/components/OfflineChip.svelte';
 	import * as m from '$lib/paraglide/messages';
-	import { ShoppingCart, CheckSquare, FileText, Search, Settings, Plus, Shield } from 'lucide-svelte';
+	import { ShoppingCart, CheckSquare, FileText, Search, Settings, Plus, Shield, Users } from 'lucide-svelte';
 
 	// Fase 12.3.1: nav items declared with their section key so we can filter
 	// by $enabledSections (precedence resolved client-side; see features.ts).
@@ -99,6 +99,19 @@
 				{label()}
 			</a>
 		{/each}
+
+		<!-- Spacer + manage-collective entry, last in nav. Always visible
+		     (no section-flag gating) since membership management is orthogonal
+		     to feature toggles. -->
+		<div class="my-2 border-t border-black/10 dark:border-white/10" aria-hidden="true"></div>
+		<a
+			href="/collective/manage"
+			data-testid="sidebar-manage-link"
+			class="mb-0.5 flex items-center gap-3 rounded-md px-3 py-2 text-sm font-medium transition-colors {activePath.startsWith('/collective/manage') ? 'bg-black/10 text-slate-900 dark:bg-white/10 dark:text-slate-50' : 'text-slate-500 hover:bg-black/5 hover:text-slate-900 dark:text-slate-400 dark:hover:bg-white/5 dark:hover:text-slate-50'}"
+		>
+			<Users size={16} strokeWidth={1.5} />
+			{m.manage_title()}
+		</a>
 	</nav>
 
 	<div class="p-3 pt-2">