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/TabletNavBar.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
 	import { page } from '$app/stores';
-	import { mobileNavItems } from './nav';
+	import { mobileNavItems, isActiveRoute } from './nav';
 </script>
 
 <nav
@@ -8,7 +8,7 @@
 	class="hidden md:flex lg:hidden gap-2 items-center p-2"
 >
 	{#each mobileNavItems 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}