chore: remove keyboard navigation UI elements and related code

- Delete static/css/components/keyboard-nav.css entirely
- Remove @import of keyboard-nav.css from style.css
- Remove keyboard-nav-hint divs from controls.html and recipes.html
- Clean up all keyboard.* translation keys from 10 locale files

The actual keyboard scrolling handlers (PageUp/PageDown in infiniteScroll.js
and VirtualScroller.js) are kept as they provide core scroll functionality.

locales/de.json

@@ -1424,15 +1424,6 @@
             "duplicate": "Dieser Tag existiert bereits"
         }
     },
-    "keyboard": {
-        "navigation": "Tastatur-Navigation:",
-        "shortcuts": {
-            "pageUp": "Eine Seite nach oben scrollen",
-            "pageDown": "Eine Seite nach unten scrollen",
-            "home": "Zum Anfang springen",
-            "end": "Zum Ende springen"
-        }
-    },
     "initialization": {
         "title": "Initialisierung",
         "message": "Ihr Arbeitsbereich wird vorbereitet...",