fix(mobile): sheets above the keyboard, emoji picker, cleaner header

- FormSheet: create dialogs (task list, shopping list, folder) as bottom
  sheet right above the on-screen keyboard (visualViewport), large fields;
  centred or anchored dialog on desktop
- Emoji picker: category icons keep their sprite size (no doubled icons,
  round highlight), one 'Zuletzt verwendet' (library category, German
  names), sheet and picker height follow the keyboard
- Mobile header no longer repeats the current section next to the logo

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -716,9 +716,6 @@ export const App: React.FC = () => {
                 </span>
                 <span className="text-[9px] text-muted font-mono leading-tight mt-0.5">{APP_VERSION}</span>
               </div>
-              <span className="text-[11px] font-semibold text-accent bg-accent/15 px-1.5 py-0.5 rounded-md">
-                {activeTabLabelMap[activeTab]}
-              </span>
               <ChevronDown
                 className={`w-4 h-4 text-muted transition-transform duration-150 ${
                   isMenuOpen ? 'rotate-180 text-accent' : ''