feat: mobile-first UI redesign (v0.15.0)

- Persistent bottom tab bar (Feed · FAB · Account) on all authenticated pages
- Upload FAB triggers bottom sheet (Galerie / Kamera) → navigates to composer
- Upload page redesigned as full-screen composer with thumbnail strip, textarea,
  quick-tag chips, sticky submit button; bottom nav suppressed while composing
- Slim upload progress bar above bottom nav driven by queue state
- Feed: list/grid view toggle; list = chronological full-width FeedListCard;
  grid = 3-col with search bar, autocomplete from loaded posts, filter chips
- Account page: role-gated dashboard links (Host / Admin); Konto section with
  leave-confirm bottom sheet; no more per-page header nav icons
- Host dashboard: back arrow, collapsible sections, 2-col stats, user search
- Admin dashboard: back arrow, inner tab bar (Stats/Config/Export/Nutzer),
  stacked config inputs with sticky save, new Nutzer tab
- BottomNav hidden on unauthenticated pages via isAuthenticated store
- FeedGrid: threeCol prop; OnboardingGuide upload step updated for FAB
- Concept docs added to docs/

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

frontend/src/lib/components/FeedGrid.svelte

@@ -6,9 +6,10 @@
 		onlike: (id: string) => void;
 		oncomment: (id: string) => void;
 		onselect: (upload: FeedUpload) => void;
+		threeCol?: boolean;
 	}
 
-	let { uploads, onlike, oncomment, onselect }: Props = $props();
+	let { uploads, onlike, oncomment, onselect, threeCol = false }: Props = $props();
 
 	function isVideo(mime: string): boolean {
 		return mime.startsWith('video/');
@@ -21,7 +22,7 @@
 	}
 </script>
 
-<div class="grid grid-cols-2 gap-2 sm:grid-cols-3">
+<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">
 			<button