feat: replace pretzel emoji with favicon across the app

Use the new favicon SVG in the header, login, setup, empty state,
and error pages. Keep the pretzel emoji in the footer tagline.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/lib/components/Layout/Header.svelte

@@ -28,6 +28,7 @@
 		</button>
 
 		<div class="flex items-center gap-2">
+			<img src="/favicon.svg" alt="" class="h-8 w-8" />
 			<h1 class="font-['Press_Start_2P'] text-lg text-[var(--primary)]">Crumbs</h1>
 		</div>