refactor: move shared components to lib/shared/ sub-packages

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

frontend/src/lib/shared/primitives/ThemeToggle.svelte

@@ -0,0 +1,74 @@
+<script lang="ts">
+import { onMount } from 'svelte';
+import { m } from '$lib/paraglide/messages.js';
+
+type Theme = 'light' | 'dark';
+
+function systemPrefersDark(): boolean {
+	return window.matchMedia('(prefers-color-scheme: dark)').matches;
+}
+
+function resolveInitialTheme(): Theme {
+	const saved = localStorage.getItem('theme');
+	if (saved === 'light' || saved === 'dark') return saved;
+	return systemPrefersDark() ? 'dark' : 'light';
+}
+
+let theme = $state<Theme>('light');
+
+onMount(() => {
+	theme = resolveInitialTheme();
+});
+
+const themeLabel = $derived(
+	theme === 'dark' ? m.theme_toggle_to_light() : m.theme_toggle_to_dark()
+);
+
+function toggle() {
+	theme = theme === 'dark' ? 'light' : 'dark';
+	localStorage.setItem('theme', theme);
+	document.documentElement.setAttribute('data-theme', theme);
+}
+</script>
+
+<button
+	type="button"
+	onclick={toggle}
+	aria-label={themeLabel}
+	title={themeLabel}
+	class="rounded p-1.5 text-white/65 transition-colors hover:bg-white/10 hover:text-white focus:outline-none focus-visible:ring-2 focus-visible:ring-focus-ring"
+>
+	{#if theme === 'dark'}
+		<!-- Sun icon — click to go light -->
+		<svg
+			class="h-5 w-5"
+			viewBox="0 0 24 24"
+			fill="none"
+			stroke="currentColor"
+			stroke-width="2"
+			aria-hidden="true"
+		>
+			<circle cx="12" cy="12" r="4" />
+			<path
+				stroke-linecap="round"
+				d="M12 2v2M12 20v2M4.22 4.22l1.42 1.42M18.36 18.36l1.42 1.42M2 12h2M20 12h2M4.22 19.78l1.42-1.42M18.36 5.64l1.42-1.42"
+			/>
+		</svg>
+	{:else}
+		<!-- Moon icon — click to go dark -->
+		<svg
+			class="h-5 w-5"
+			viewBox="0 0 24 24"
+			fill="none"
+			stroke="currentColor"
+			stroke-width="2"
+			aria-hidden="true"
+		>
+			<path
+				stroke-linecap="round"
+				stroke-linejoin="round"
+				d="M21 12.79A9 9 0 1111.21 3 7 7 0 0021 12.79z"
+			/>
+		</svg>
+	{/if}
+</button>