refactor(layout): extract AppNav and UserMenu sub-components

Split +layout.svelte (205 lines) into:
- AppNav.svelte: logo + nav links with active-state styling
- UserMenu.svelte: avatar button, dropdown, click-outside handler

Layout drops from 205 → 80 lines.

Part of #75

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

frontend/src/routes/AppNav.svelte

@@ -0,0 +1,59 @@
+<script lang="ts">
+import { page } from '$app/state';
+import { m } from '$lib/paraglide/messages.js';
+
+let { isAdmin = false }: { isAdmin?: boolean } = $props();
+</script>
+
+<div class="flex">
+	<div class="mr-10 flex flex-shrink-0 items-center">
+		<a href="/" class="flex items-center" aria-label="Familienarchiv">
+			<span class="font-sans text-xl font-bold tracking-widest text-ink uppercase"
+				>Familienarchiv</span
+			>
+		</a>
+	</div>
+
+	<nav class="hidden items-center sm:flex sm:space-x-1">
+		<a
+			href="/"
+			class="inline-flex items-center px-3 py-1.5 font-sans text-xs font-bold tracking-widest uppercase transition-colors
+                   {page.url.pathname === '/' || page.url.pathname.startsWith('/documents')
+                    ? 'rounded bg-nav-active text-ink'
+                    : 'rounded text-ink-2 hover:bg-muted hover:text-ink'}"
+		>
+			{m.nav_documents()}
+		</a>
+
+		<a
+			href="/persons"
+			class="inline-flex items-center px-3 py-1.5 font-sans text-xs font-bold tracking-widest uppercase transition-colors
+                   {page.url.pathname.startsWith('/persons')
+                    ? 'rounded bg-nav-active text-ink'
+                    : 'rounded text-ink-2 hover:bg-muted hover:text-ink'}"
+		>
+			{m.nav_persons()}
+		</a>
+
+		<a
+			href="/conversations"
+			class="inline-flex items-center px-3 py-1.5 font-sans text-xs font-bold tracking-widest uppercase transition-colors
+                   {page.url.pathname.startsWith('/conversations')
+                    ? 'rounded bg-nav-active text-ink'
+                    : 'rounded text-ink-2 hover:bg-muted hover:text-ink'}"
+		>
+			{m.nav_conversations()}
+		</a>
+		{#if isAdmin}
+			<a
+				href="/admin"
+				class="inline-flex items-center px-3 py-1.5 font-sans text-xs font-bold tracking-widest uppercase transition-colors
+                   {page.url.pathname.startsWith('/admin')
+                        ? 'rounded bg-nav-active text-ink'
+                        : 'rounded text-ink-2 hover:bg-muted hover:text-ink'}"
+			>
+				{m.nav_admin()}
+			</a>
+		{/if}
+	</nav>
+</div>