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/routes/+layout.svelte

@@ -3,9 +3,22 @@
 	import '../app.css';
 	import { initAuth } from '$lib/auth';
 	import { onMount } from 'svelte';
+	import BottomNav from '$lib/components/BottomNav.svelte';
+	import UploadSheet from '$lib/components/UploadSheet.svelte';
+	import { showBottomNav } from '$lib/ui-store';
+	import { isAuthenticated } from '$lib/auth';
+	import { queueItems, isProcessing } from '$lib/upload-queue';
 
 	let { children } = $props();
 
+	// Slim progress bar: ratio of completed items to total, shown while processing.
+	let progressPct = $derived.by(() => {
+		const total = $queueItems.length;
+		if (total === 0) return 0;
+		const done = $queueItems.filter((i) => i.status === 'done').length;
+		return Math.round((done / total) * 100);
+	});
+
 	onMount(() => {
 		initAuth();
 	});
@@ -16,3 +29,23 @@
 </svelte:head>
 
 {@render children()}
+
+<!-- Slim upload progress bar — sits just above the bottom nav -->
+{#if $isProcessing && $isAuthenticated && $showBottomNav}
+	<div
+		class="fixed z-30 h-0.5 bg-gray-200 transition-all"
+		style="bottom: calc(3.5rem + env(safe-area-inset-bottom)); left: 0; right: 0"
+	>
+		<div
+			class="h-full bg-blue-500 transition-all duration-500"
+			style="width: {progressPct}%"
+		></div>
+	</div>
+{/if}
+
+<!-- UploadSheet is always mounted for smooth enter/exit animation -->
+<UploadSheet />
+
+{#if $showBottomNav && $isAuthenticated}
+	<BottomNav />
+{/if}