feat(ui): v0.16 features + dark mode across every page

Wires up everything from the previous commits into actual UI surfaces, and
applies Tailwind dark: variants throughout. All pages now support the
'system' / 'light' / 'dark' preference set in the onboarding step or in
Mein Konto → Design.

Layout & nav:
- routes/+layout.svelte: initTheme(), global pin-reset SSE handler that
  filters by user_id and calls clearPin(), one-shot /me/context fetch
  on boot to hydrate privacyNote + quota.
- components/BottomNav.svelte: dark variants on the frosted-glass bar.
- components/UploadSheet.svelte: dark variants on backdrop, sheet,
  source buttons.
- components/OnboardingGuide.svelte: new "Helles oder dunkles Design?"
  step (3-option custom-radio grid), reactive currentStep with proper
  type narrowing, dark variants throughout. Privacy-note nudge appears
  on the PIN step only when one is configured.

Feed:
- routes/feed/+page.svelte: diashow entry icon (tablet/desktop only),
  long-press → ContextSheet (Löschen for own posts, Original anzeigen
  for all), upload-deleted + feed-delta SSE handlers, dark variants on
  header, search, autocomplete, filter chips, empty states.
- components/FeedListCard.svelte: long-press wireup, double-tap-to-like,
  data-mode-aware mediaSrc via pickMediaUrl, kebab fallback for desktop,
  isOwn prop, dark variants.
- components/FeedGrid.svelte: long-press wireup, dark variants.
- components/LightboxModal.svelte: data-mode-aware src, double-tap heart
  burst, dark variants on card / comments / input.
- components/HashtagChips.svelte: dark variants.

Account:
- routes/account/+page.svelte: theme picker (3-button radio grid), data
  mode picker (with confirm sheet for Original), live quota widget,
  preformatted Datenschutzhinweis block, diashow tile (mobile only),
  pin now sourced from the $currentPin store so a global pin-reset
  clears it live, clearQueue() on explicit logout, dark variants
  across every card + both bottom sheets.

Upload:
- routes/upload/+page.svelte: per-user quota progress bar above the
  submit button, dark variants.

Host & Admin:
- routes/host/+page.svelte: PIN-reset confirm + one-time PIN modal,
  hosts may demote other hosts, canResetPinFor() helper, dark variants
  on all cards, modals, stats, toast.
- routes/admin/+page.svelte: Config form rebuilt as CONFIG_GROUPS with
  per-field kind (number / bool / text), renders toggles for the
  rate-limit + quota switches and a textarea for the privacy_note;
  Nutzer tab gains PIN reset + hosts-may-demote-hosts wiring; same
  one-time PIN modal; dark variants everywhere.
- routes/admin/login/+page.svelte: dark variants.

Join / Recover / Export:
- routes/join/+page.svelte: rename inline link to
  "Ich habe bereits einen Account", dark variants.
- routes/recover/+page.svelte: dark variants.
- routes/export/+page.svelte: dark variants on status cards + HTML
  guide modal.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/FeedGrid.svelte

@@ -1,30 +1,40 @@
 <script lang="ts">
 	import type { FeedUpload } from '$lib/types';
+	import { dataMode } from '$lib/data-mode-store';
+	import { longpress } from '$lib/actions/longpress';
 
 	interface Props {
 		uploads: FeedUpload[];
 		onlike: (id: string) => void;
 		oncomment: (id: string) => void;
 		onselect: (upload: FeedUpload) => void;
+		oncontextmenu?: (upload: FeedUpload) => void;
 		threeCol?: boolean;
 	}
 
-	let { uploads, onlike, oncomment, onselect, threeCol = false }: Props = $props();
+	let { uploads, onlike, oncomment, onselect, oncontextmenu, threeCol = false }: Props =
+		$props();
 
 	function isVideo(mime: string): boolean {
 		return mime.startsWith('video/');
 	}
 
-	function imageUrl(upload: FeedUpload): string {
+	// Grid uses small thumbnails by design even in Original mode — full media is one tap
+	// away in the lightbox, where the data-mode picker decides for real.
+	function tileUrl(upload: FeedUpload): string {
 		if (upload.thumbnail_url) return upload.thumbnail_url;
 		if (upload.preview_url) return upload.preview_url;
-		return '';
+		return $dataMode === 'original' ? `/api/v1/upload/${upload.id}/original` : '';
 	}
 </script>
 
 <div class="grid gap-0.5 {threeCol ? 'grid-cols-3' : 'grid-cols-2 sm:grid-cols-3'}">
 	{#each uploads as upload (upload.id)}
-		<div class="group relative aspect-square cursor-pointer overflow-hidden rounded-lg bg-gray-100">
+		<div
+			class="group relative aspect-square cursor-pointer overflow-hidden rounded-lg bg-gray-100 dark:bg-gray-800"
+			use:longpress={{ duration: 500 }}
+			onlongpress={() => oncontextmenu?.(upload)}
+		>
 			<button
 				onclick={() => onselect(upload)}
 				class="block h-full w-full"
@@ -32,8 +42,8 @@
 			>
 				{#if isVideo(upload.mime_type)}
 					<div class="flex h-full items-center justify-center bg-gray-800">
-						{#if imageUrl(upload)}
-							<img src={imageUrl(upload)} alt="" class="h-full w-full object-cover" />
+						{#if tileUrl(upload)}
+							<img src={tileUrl(upload)} alt="" class="h-full w-full object-cover" />
 						{/if}
 						<div class="absolute inset-0 flex items-center justify-center">
 							<svg class="h-10 w-10 text-white/80" fill="currentColor" viewBox="0 0 24 24">
@@ -41,8 +51,8 @@
 							</svg>
 						</div>
 					</div>
-				{:else if imageUrl(upload)}
-					<img src={imageUrl(upload)} alt="" class="h-full w-full object-cover" loading="lazy" />
+				{:else if tileUrl(upload)}
+					<img src={tileUrl(upload)} alt="" class="h-full w-full object-cover" loading="lazy" />
 				{:else}
 					<div class="flex h-full items-center justify-center text-gray-400">
 						<svg class="h-8 w-8" fill="none" viewBox="0 0 24 24" stroke="currentColor">