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/lib/__tests__/useKeyboardInset.test.tsx

@@ -0,0 +1,35 @@
+import { describe, it, expect } from 'vitest';
+import { act, renderHook } from '@testing-library/react';
+import { useKeyboardInset } from '../useKeyboardInset';
+
+// Simulates iOS: the layout viewport keeps its height, the visual viewport shrinks by the keyboard
+function mockVisualViewport(height: number) {
+  const listeners: Record<string, (() => void)[]> = {};
+  const vv = {
+    height,
+    offsetTop: 0,
+    addEventListener: (t: string, fn: () => void) => (listeners[t] = [...(listeners[t] || []), fn]),
+    removeEventListener: (t: string, fn: () => void) => (listeners[t] = (listeners[t] || []).filter((f) => f !== fn)),
+  };
+  Object.defineProperty(window, 'visualViewport', { value: vv, configurable: true });
+  Object.defineProperty(window, 'innerHeight', { value: 844, configurable: true });
+  return {
+    open(keyboard: number) {
+      vv.height = 844 - keyboard;
+      (listeners.resize || []).forEach((fn) => fn());
+    },
+  };
+}
+
+describe('useKeyboardInset', () => {
+  it('reports the keyboard height so sheets can sit above it', () => {
+    const vv = mockVisualViewport(844);
+    const { result } = renderHook(() => useKeyboardInset());
+    expect(result.current.inset).toBe(0);
+    act(() => vv.open(336));
+    expect(result.current.inset).toBe(336);
+    expect(result.current.visibleHeight).toBe(508);
+    act(() => vv.open(0));
+    expect(result.current.inset).toBe(0);
+  });
+});