feat: app-wide toast notifications

Adds a client-side toast store (info/success/error, auto-dismiss with a
sticky option) and a Toaster component mounted in the root layout, wired to
the reserved --z-toast token. Gives mutations a consistent way to surface
success/failure instead of inline-only text or silent catches.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/routes/+layout.svelte

@@ -10,6 +10,7 @@
     import AppBar from '$lib/components/AppBar.svelte';
     import BottomNav, { type BottomNavTab } from '$lib/components/BottomNav.svelte';
     import NavProgress from '$lib/components/NavProgress.svelte';
+    import Toaster from '$lib/components/Toaster.svelte';
     import IconButton from '$lib/components/IconButton.svelte';
     import Upload from '@lucide/svelte/icons/upload';
     import UserCircle from '@lucide/svelte/icons/user-circle';
@@ -271,6 +272,8 @@
     </div>
 {/if}
 
+<Toaster />
+
 <style>
     header {
         padding: var(--space-3) var(--space-4);