feat(fase-16): integrate Spinner at the 5 loading sites + E2E coverage

Fase 16.2 + 16.3 — replace the bare `m.loading()` text at every
pre-existing async-loading site with `<Spinner size=...> + <span>`.
Text is kept (visual fallback + tooltip / SR redundancy). No new
loading semantics — only existing flags get a visual indicator.

Sites:
- (app)/+layout.svelte: auth splash gets `size="lg"` centred under the
  loading label.
- (app)/search/+page.svelte: inline `size="sm"` next to the search-in-flight
  copy.
- (app)/notes/archive/+page.svelte: inline `size="sm"` next to the
  archive-load copy.
- lib/components/CreateCollectiveModal.svelte: inline `size="sm"` inside
  the disabled Save button while the create RPC is in flight.
- (app)/collective/manage/+page.svelte: three sites — members-list load,
  invitation-generate button, and add-common-item Save button.

E2E (tests/e2e/spinner.test.ts):
- SP-E-01 throttles search_in_collective RPC by 500ms and asserts the
  spinner is visible in the loading paragraph.
- SP-E-02 throttles set_item_frequency_weight RPC and asserts the spinner
  is scoped inside the Save button.
- SP-E-03 seeds an expired `sb-192-auth-token` so GoTrue is forced to
  call /auth/v1/token, then hangs that endpoint indefinitely and
  asserts the splash spinner is visible. Storage key derivation matches
  Supabase's `sb-${hostname.split('.')[0]}-auth-token` default; we
  populate both the LAN-IP variant and a loopback fallback for
  resilience.

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

apps/web/src/lib/components/CreateCollectiveModal.svelte

@@ -2,6 +2,7 @@
 	import { goto } from '$app/navigation';
 	import { getSupabase } from '$lib/supabase';
 	import { currentCollective, userCollectives } from '$lib/stores/collective';
+	import Spinner from './Spinner.svelte';
 	import * as m from '$lib/paraglide/messages';
 
 	let { onClose }: { onClose: () => void } = $props();
@@ -102,11 +103,16 @@
 					type="submit"
 					data-testid="create-collective-modal-submit"
 					disabled={!name.trim() || creating}
-					class="rounded-lg bg-slate-900 px-4 py-1.5 text-sm font-semibold text-white
+					class="inline-flex items-center gap-2 rounded-lg bg-slate-900 px-4 py-1.5 text-sm font-semibold text-white
 						transition-opacity hover:opacity-90 disabled:opacity-50
 						dark:bg-slate-50 dark:text-slate-900"
 				>
-					{creating ? m.loading() : m.create_collective_modal_button()}
+					{#if creating}
+						<Spinner size="sm" />
+						<span>{m.loading()}</span>
+					{:else}
+						{m.create_collective_modal_button()}
+					{/if}
 				</button>
 			</div>
 		</form>