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/ja.json

@@ -1424,15 +1424,6 @@
             "duplicate": "このタグは既に存在します"
         }
     },
-    "keyboard": {
-        "navigation": "キーボードナビゲーション：",
-        "shortcuts": {
-            "pageUp": "1ページ上にスクロール",
-            "pageDown": "1ページ下にスクロール",
-            "home": "トップにジャンプ",
-            "end": "ボトムにジャンプ"
-        }
-    },
     "initialization": {
         "title": "初期化中",
         "message": "ワークスペースを準備中...",