feat: implement onboarding guide and HTML export guide

Add 4-step dismissible onboarding overlay shown on first feed visit
(welcome, upload, hashtags, PIN importance). Dismissed state persisted
in localStorage under eventsnap_guide_seen. Step indicator dots and
skip/continue buttons included.

Update HTML export guide modal to persist the eventsnap_html_guide_seen
flag: first download shows the instructions modal; subsequent clicks go
straight to download without interruption.

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

frontend/src/lib/components/OnboardingGuide.svelte

@@ -0,0 +1,85 @@
+<script lang="ts">
+	import { browser } from '$app/environment';
+
+	const GUIDE_SEEN_KEY = 'eventsnap_guide_seen';
+
+	let visible = $state(false);
+	let step = $state(0);
+
+	const steps = [
+		{
+			icon: '📸',
+			title: 'Willkommen bei EventSnap!',
+			body: 'Hier kannst du Fotos und Videos mit allen Gästen teilen — in Echtzeit, ganz ohne App-Store.'
+		},
+		{
+			icon: '⬆️',
+			title: 'Fotos & Videos hochladen',
+			body: 'Tippe oben auf „Hochladen", um Fotos aus deiner Galerie oder direkt mit der Kamera aufzunehmen. Mehrere Dateien auf einmal sind kein Problem!'
+		},
+		{
+			icon: '#️⃣',
+			title: 'Hashtags nutzen',
+			body: 'Füge in deiner Bildunterschrift #hashtags ein, um Fotos zu gruppieren — z.B. #tanz, #buffet oder #reden. Du kannst danach filtern.'
+		},
+		{
+			icon: '🔑',
+			title: 'Deinen PIN merken!',
+			body: 'Du hast beim Registrieren einen 4-stelligen PIN erhalten. Speichere ihn — du brauchst ihn, um dein Konto auf einem anderen Gerät wiederherzustellen. Er ist immer unter „Mein Konto" zu finden.'
+		}
+	];
+
+	if (browser && !localStorage.getItem(GUIDE_SEEN_KEY)) {
+		visible = true;
+	}
+
+	function next() {
+		if (step < steps.length - 1) {
+			step++;
+		} else {
+			dismiss();
+		}
+	}
+
+	function dismiss() {
+		if (browser) localStorage.setItem(GUIDE_SEEN_KEY, '1');
+		visible = false;
+	}
+</script>
+
+{#if visible}
+	<!-- Backdrop -->
+	<div class="fixed inset-0 z-50 flex items-end justify-center bg-black/60 sm:items-center">
+		<div class="w-full max-w-sm rounded-t-3xl bg-white p-6 shadow-2xl sm:rounded-2xl">
+			<!-- Step indicator -->
+			<div class="mb-5 flex justify-center gap-1.5">
+				{#each steps as _, i}
+					<div class="h-1.5 rounded-full transition-all {i === step ? 'w-6 bg-blue-600' : 'w-1.5 bg-gray-200'}"></div>
+				{/each}
+			</div>
+
+			<!-- Content -->
+			<div class="mb-6 text-center">
+				<div class="mb-3 text-5xl">{steps[step].icon}</div>
+				<h2 class="mb-2 text-xl font-bold text-gray-900">{steps[step].title}</h2>
+				<p class="text-sm leading-relaxed text-gray-600">{steps[step].body}</p>
+			</div>
+
+			<!-- Buttons -->
+			<div class="flex gap-2">
+				<button
+					onclick={dismiss}
+					class="flex-1 rounded-xl border border-gray-200 py-3 text-sm text-gray-500 hover:bg-gray-50"
+				>
+					Überspringen
+				</button>
+				<button
+					onclick={next}
+					class="flex-1 rounded-xl bg-blue-600 py-3 text-sm font-semibold text-white hover:bg-blue-700"
+				>
+					{step < steps.length - 1 ? 'Weiter' : 'Los geht\'s!'}
+				</button>
+			</div>
+		</div>
+	</div>
+{/if}

frontend/src/routes/export/+page.svelte

@@ -16,6 +16,8 @@
 		html: JobStatus;
 	}
 
+	const HTML_GUIDE_KEY = 'eventsnap_html_guide_seen';
+
 	let status = $state<ExportStatus | null>(null);
 	let showHtmlGuide = $state(false);
 	let loading = $state(true);
@@ -75,10 +77,15 @@
 	}
 
 	function downloadHtml() {
+		if (localStorage.getItem(HTML_GUIDE_KEY)) {
+			window.location.href = '/api/v1/export/html';
+		} else {
 			showHtmlGuide = true;
 		}
+	}
 
 	function confirmHtmlDownload() {
+		localStorage.setItem(HTML_GUIDE_KEY, '1');
 		showHtmlGuide = false;
 		window.location.href = '/api/v1/export/html';
 	}

frontend/src/routes/feed/+page.svelte

@@ -7,6 +7,7 @@
 	import FeedGrid from '$lib/components/FeedGrid.svelte';
 	import HashtagChips from '$lib/components/HashtagChips.svelte';
 	import LightboxModal from '$lib/components/LightboxModal.svelte';
+	import OnboardingGuide from '$lib/components/OnboardingGuide.svelte';
 	import type { FeedUpload, FeedResponse, HashtagCount } from '$lib/types';
 
 	let uploads = $state<FeedUpload[]>([]);
@@ -223,3 +224,6 @@
 		onlike={handleLike}
 	/>
 {/if}
+
+<!-- First-visit onboarding guide -->
+<OnboardingGuide />