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/BottomNav.svelte

@@ -9,7 +9,7 @@
 
 <!-- Bottom navigation bar — fixed, full-width, safe-area aware -->
 <nav
-	class="fixed bottom-0 left-0 right-0 z-40 border-t border-gray-200 bg-white/90 backdrop-blur-md"
+	class="fixed bottom-0 left-0 right-0 z-40 border-t border-gray-200 bg-white/90 backdrop-blur-md dark:border-gray-800 dark:bg-gray-900/90"
 	style="padding-bottom: env(safe-area-inset-bottom)"
 >
 	<div class="mx-auto flex h-14 max-w-2xl items-end justify-around px-4 pb-1">
@@ -17,7 +17,7 @@
 		<a
 			href="/feed"
 			class="flex flex-col items-center gap-0.5 px-4 py-1 text-xs font-medium transition-colors
-				{isActive('/feed') ? 'text-blue-600' : 'text-gray-400 hover:text-gray-600'}"
+				{isActive('/feed') ? 'text-blue-600 dark:text-blue-400' : 'text-gray-400 hover:text-gray-600 dark:text-gray-500 dark:hover:text-gray-300'}"
 			aria-label="Galerie"
 		>
 			<svg class="h-6 w-6" fill="none" viewBox="0 0 24 24" stroke="currentColor" stroke-width="1.5">
@@ -53,7 +53,7 @@
 		<a
 			href="/account"
 			class="flex flex-col items-center gap-0.5 px-4 py-1 text-xs font-medium transition-colors
-				{isActive('/account') ? 'text-blue-600' : 'text-gray-400 hover:text-gray-600'}"
+				{isActive('/account') ? 'text-blue-600 dark:text-blue-400' : 'text-gray-400 hover:text-gray-600 dark:text-gray-500 dark:hover:text-gray-300'}"
 			aria-label="Konto"
 		>
 			<svg class="h-6 w-6" fill="none" viewBox="0 0 24 24" stroke="currentColor" stroke-width="1.5">