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/index.css

@@ -484,7 +484,10 @@ body {
     min-height: 44px !important;
   }
 
-  .mindsift-emoji-picker .EmojiPickerReact .epr-category-nav button {
-    min-height: 44px !important;
+  /* Category icons are sprite cut-outs: they must keep the library's square size (stretching them showed
+     two icons and an oval highlight). More breathing room comes from the nav row instead. */
+  .mindsift-emoji-picker .EmojiPickerReact .epr-category-nav {
+    padding-top: 6px !important;
+    padding-bottom: 6px !important;
   }
 }