fix(nav): use segment-boundary route matching to prevent false positives

Extracts isActiveRoute() into shared nav module. Matches exact path
or path + '/' prefix, preventing /settings from matching /settings-advanced.

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

frontend/src/lib/nav/DesktopSidebar.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
 	import { page } from '$app/stores';
-	import { desktopNavSections } from './nav';
+	import { desktopNavSections, isActiveRoute } from './nav';
 
 	let { appName, householdName }: { appName: string; householdName: string } = $props();
 </script>
@@ -24,7 +24,7 @@
 				{section.title}
 			</p>
 			{#each section.items as item (item.href)}
-				{@const active = $page.url.pathname.startsWith(item.href)}
+				{@const active = isActiveRoute(item.href, $page.url.pathname)}
 				<a
 					href={item.href}
 					aria-current={active ? 'page' : undefined}