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/pending-upload-store.ts

@@ -0,0 +1,17 @@
+import { writable } from 'svelte/store';
+
+export interface PendingFile {
+	file: File;
+	previewUrl: string; // URL.createObjectURL result — revoke after use
+}
+
+export const pendingFiles = writable<PendingFile[]>([]);
+export const pendingCaption = writable('');
+
+export function clearPending() {
+	pendingFiles.update((files) => {
+		for (const f of files) URL.revokeObjectURL(f.previewUrl);
+		return [];
+	});
+	pendingCaption.set('');
+}