refactor(auth): centralize appReady pattern into AuthGate component

Replace copy-pasted appReady/loading/redirect logic in all 13 layouts
with a shared AuthGate component. Supports guest mode, onReady callback
for app-specific data loading, and configurable login redirect.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/shared-auth-ui/src/components/AuthGate.svelte

@@ -0,0 +1,86 @@
+<!--
+  AuthGate - Centralized auth initialization and loading gate.
+
+  Handles:
+  - Auth store initialization
+  - Loading spinner while checking auth
+  - Redirect to login if not authenticated (unless allowGuest)
+  - Calling onReady callback after auth is confirmed
+  - Rendering children only when ready
+
+  Usage:
+    <AuthGate authStore={authStore} onReady={loadAppData}>
+      <AppContent />
+    </AuthGate>
+-->
+<script lang="ts">
+	import { onMount } from 'svelte';
+	import type { Snippet } from 'svelte';
+
+	/**
+	 * Minimal interface that all app auth stores must satisfy.
+	 * Every app's authStore (e.g. `$lib/stores/auth.svelte`) already matches this.
+	 */
+	interface AuthStoreInterface {
+		initialize(): Promise<void>;
+		readonly isAuthenticated: boolean;
+	}
+
+	interface Props {
+		/** The app's auth store instance (must have initialize() and isAuthenticated) */
+		authStore: AuthStoreInterface;
+		/** Path to redirect to when not authenticated (default: '/login') */
+		loginHref?: string;
+		/** If true, render children even when not authenticated (for guest-mode apps) */
+		allowGuest?: boolean;
+		/** Callback invoked after auth is confirmed, before children are rendered.
+		 *  Use this for loading app-specific data (projects, calendars, etc.) */
+		onReady?: () => void | Promise<void>;
+		/** SvelteKit goto function for client-side navigation. Falls back to window.location. */
+		goto?: (url: string, opts?: Record<string, unknown>) => unknown;
+		/** Content to render when ready */
+		children: Snippet;
+	}
+
+	let {
+		authStore,
+		loginHref = '/login',
+		allowGuest = false,
+		onReady,
+		goto: gotoFn,
+		children,
+	}: Props = $props();
+
+	let ready = $state(false);
+
+	function navigate(url: string) {
+		if (gotoFn) {
+			gotoFn(url);
+		} else if (typeof window !== 'undefined') {
+			window.location.href = url;
+		}
+	}
+
+	onMount(async () => {
+		await authStore.initialize();
+
+		if (!authStore.isAuthenticated && !allowGuest) {
+			navigate(loginHref);
+			return;
+		}
+
+		if (onReady) {
+			await onReady();
+		}
+
+		ready = true;
+	});
+</script>
+
+{#if !ready}
+	<div class="flex items-center justify-center h-screen bg-background">
+		<div class="animate-spin rounded-full h-8 w-8 border-b-2 border-primary"></div>
+	</div>
+{:else}
+	{@render children()}
+{/if}