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/ui-store.ts

@@ -0,0 +1,13 @@
+import { writable, derived } from 'svelte/store';
+import { queueItems } from './upload-queue';
+
+// Controls BottomNav visibility. Upload page sets this false on mount and restores on destroy.
+export const showBottomNav = writable(true);
+
+// Controls the UploadSheet overlay. FAB sets true; sheet sets false.
+export const uploadSheetOpen = writable(false);
+
+// Count of items currently pending or uploading — shown as FAB badge.
+export const uploadBadgeCount = derived(queueItems, ($items) =>
+	$items.filter((i) => i.status === 'pending' || i.status === 'uploading').length
+);