diff --git a/apps/web/src/lib/components/Spinner.svelte b/apps/web/src/lib/components/Spinner.svelte
new file mode 100644
index 0000000..cee78eb
--- /dev/null
+++ b/apps/web/src/lib/components/Spinner.svelte
@@ -0,0 +1,34 @@
+<script lang="ts">
+	import * as m from '$lib/paraglide/messages';
+
+	interface Props {
+		size?: 'sm' | 'md' | 'lg';
+		class?: string;
+	}
+	let { size = 'md', class: extra = '' }: Props = $props();
+
+	const px = $derived(size === 'sm' ? 16 : size === 'lg' ? 40 : 24);
+</script>
+
+<div role="status" aria-live="polite" class={extra}>
+	<svg
+		data-testid="spinner"
+		width={px}
+		height={px}
+		viewBox="0 0 24 24"
+		fill="none"
+		class="animate-spin motion-reduce:animate-none"
+	>
+		<circle
+			cx="12"
+			cy="12"
+			r="9"
+			stroke="currentColor"
+			stroke-width="2.5"
+			stroke-linecap="round"
+			stroke-dasharray="42 14"
+			opacity="0.75"
+		/>
+	</svg>
+	<span class="sr-only">{m.loading()}</span>
+</div>
diff --git a/apps/web/src/lib/components/Spinner.test.ts b/apps/web/src/lib/components/Spinner.test.ts
new file mode 100644
index 0000000..3e101f9
--- /dev/null
+++ b/apps/web/src/lib/components/Spinner.test.ts
@@ -0,0 +1,79 @@
+/**
+ * Fase 16 — Spinner component unit tests.
+ *
+ * Vitest + jsdom + Svelte 5's `mount()` API. There is no @testing-library/svelte
+ * in the repo (yet), but Svelte 5 exposes `mount`/`unmount` directly so we can
+ * inspect the rendered DOM without an extra dependency.
+ */
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
+import { mount, unmount } from 'svelte';
+import Spinner from './Spinner.svelte';
+import * as m from '$lib/paraglide/messages';
+
+let host: HTMLElement;
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+let instance: any;
+
+function render(props: Record<string, unknown> = {}) {
+	host = document.createElement('div');
+	document.body.appendChild(host);
+	instance = mount(Spinner, { target: host, props });
+	return host;
+}
+
+beforeEach(() => {
+	host = document.createElement('div');
+});
+
+afterEach(() => {
+	if (instance) unmount(instance);
+	if (host?.parentNode) host.parentNode.removeChild(host);
+});
+
+describe('Spinner component', () => {
+	it('SP-U-01: default size renders a 24x24 SVG', () => {
+		const root = render();
+		const svg = root.querySelector('svg[data-testid="spinner"]');
+		expect(svg).not.toBeNull();
+		expect(svg?.getAttribute('width')).toBe('24');
+		expect(svg?.getAttribute('height')).toBe('24');
+	});
+
+	it("SP-U-02: size='sm' renders 16x16 and size='lg' renders 40x40", () => {
+		const smRoot = render({ size: 'sm' });
+		const smSvg = smRoot.querySelector('svg[data-testid="spinner"]');
+		expect(smSvg?.getAttribute('width')).toBe('16');
+		expect(smSvg?.getAttribute('height')).toBe('16');
+		unmount(instance);
+		instance = null;
+		smRoot.remove();
+
+		const lgRoot = render({ size: 'lg' });
+		const lgSvg = lgRoot.querySelector('svg[data-testid="spinner"]');
+		expect(lgSvg?.getAttribute('width')).toBe('40');
+		expect(lgSvg?.getAttribute('height')).toBe('40');
+	});
+
+	it('SP-U-03: outer wrapper exposes role="status" and aria-live="polite"', () => {
+		const root = render();
+		const wrapper = root.querySelector('[role="status"]');
+		expect(wrapper).not.toBeNull();
+		expect(wrapper?.getAttribute('aria-live')).toBe('polite');
+	});
+
+	it("SP-U-04: includes an sr-only label sourced from Paraglide m.loading()", () => {
+		const root = render();
+		const srOnly = root.querySelector('.sr-only');
+		expect(srOnly).not.toBeNull();
+		// Paraglide's English default is "Loading…" — we don't hard-code it here,
+		// we compare against the message itself so an i18n switch doesn't break.
+		expect(srOnly?.textContent).toBe(m.loading());
+	});
+
+	it('SP-U-05: forwards the `class` prop onto the wrapper element', () => {
+		const root = render({ class: 'text-amber-500 ml-2' });
+		const wrapper = root.querySelector('[role="status"]');
+		expect(wrapper?.className).toContain('text-amber-500');
+		expect(wrapper?.className).toContain('ml-2');
+	});
+});
diff --git a/apps/web/vitest.config.ts b/apps/web/vitest.config.ts
index a6c1733..9760ef3 100644
--- a/apps/web/vitest.config.ts
+++ b/apps/web/vitest.config.ts
@@ -16,9 +16,20 @@ export default defineConfig({
 		globals: true,
 		include: ['src/**/*.test.ts'],
 		exclude: ['tests/**', 'node_modules/**', '.svelte-kit/**'],
-		setupFiles: ['./vitest.setup.ts']
+		setupFiles: ['./vitest.setup.ts'],
+		// Force Svelte to load its browser entry (index-client.js) instead of
+		// the server entry — `mount()` only exists on the client. Without this
+		// any test that mounts a Svelte 5 component throws
+		// `lifecycle_function_unavailable`. jsdom + the `browser` condition is
+		// the canonical Svelte 5 unit-test setup.
+		server: {
+			deps: {
+				inline: [/^svelte/]
+			}
+		}
 	},
 	resolve: {
+		conditions: ['browser'],
 		alias: {
 			$lib: new URL('./src/lib', import.meta.url).pathname
 		}