feat(fase-10): sidebar "+ New collective" entry + modal (10.6)

Adds a new "+ New collective" entry at the bottom of the sidebar
collective-switcher dropdown (and the mobile drawer for parity).
Clicking it opens a CreateCollectiveModal that reuses the same
onboarding form — name + emoji grid — and calls the existing
create_collective() RPC (migration 012). On success, the new
collective is appended to $userCollectives, set active in
$currentCollective, persisted to localStorage, and the user is sent
to /lists.

Side effects:
- Switcher now opens even with a single collective (previously gated
  on >1) so the entry is always reachable; the chevron is always
  shown.
- DesktopSidebar gets a data-testid on the switcher button so the
  Playwright suite can target it without text matching.

O-04 (rescued from Fase 7): Eva creates her first collective via
onboarding, then opens the switcher and creates a second one without
leaving /lists; both collectives end up in the switcher and the DB.

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

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

@@ -3,8 +3,9 @@
 	import { displayName } from '$lib/stores/auth';
 	import { currentCollective, userCollectives } from '$lib/stores/collective';
 	import Avatar from '$lib/components/Avatar.svelte';
+	import CreateCollectiveModal from '$lib/components/CreateCollectiveModal.svelte';
 	import * as m from '$lib/paraglide/messages';
-	import { ShoppingCart, CheckSquare, FileText, Search, Settings } from 'lucide-svelte';
+	import { ShoppingCart, CheckSquare, FileText, Search, Settings, Plus } from 'lucide-svelte';
 
 	const navItems = [
 		{ href: '/lists',  label: () => m.nav_lists(),  icon: ShoppingCart },
@@ -14,6 +15,7 @@
 	];
 
 	let switcherOpen = $state(false);
+	let showCreate = $state(false);
 
 	const activePath = $derived($page.url.pathname);
 
@@ -25,6 +27,11 @@
 			switcherOpen = false;
 		}
 	}
+
+	function openCreateCollective() {
+		switcherOpen = false;
+		showCreate = true;
+	}
 </script>
 
 <aside
@@ -33,6 +40,7 @@
 >
 	<div class="relative">
 		<button
+			data-testid="sidebar-collective-switcher"
 			class="flex h-14 w-full items-center gap-2 px-4 text-left hover:bg-black/5 dark:hover:bg-white/5"
 			onclick={() => (switcherOpen = !switcherOpen)}
 			aria-expanded={switcherOpen}
@@ -41,11 +49,9 @@
 			<span class="min-w-0 flex-1 truncate text-sm font-semibold text-slate-900 dark:text-slate-50">
 				{$currentCollective?.name ?? m.app_name()}
 			</span>
-			{#if $userCollectives.length > 1}
-				<span class="text-xs text-slate-400">⌄</span>
-			{/if}
+			<span class="text-xs text-slate-400">⌄</span>
 		</button>
-		{#if switcherOpen && $userCollectives.length > 1}
+		{#if switcherOpen}
 			<div class="absolute left-0 right-0 top-full z-50 bg-surface-raised shadow-[0px_20px_40px_rgba(15,23,42,0.06)]">
 				{#each $userCollectives as c}
 					<button
@@ -56,6 +62,15 @@
 						<span class="truncate">{c.name}</span>
 					</button>
 				{/each}
+				<button
+					data-testid="sidebar-create-collective"
+					class="flex w-full items-center gap-2 border-t border-black/5 px-4 py-2.5 text-left text-sm font-medium text-slate-700
+						hover:bg-black/5 dark:border-white/5 dark:text-slate-300 dark:hover:bg-white/5"
+					onclick={openCreateCollective}
+				>
+					<Plus size={14} strokeWidth={1.5} />
+					{m.sidebar_create_collective()}
+				</button>
 			</div>
 		{/if}
 	</div>
@@ -90,3 +105,7 @@
 		</div>
 	</div>
 </aside>
+
+{#if showCreate}
+	<CreateCollectiveModal onClose={() => (showCreate = false)} />
+{/if}