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/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 />