Refactor fullscreen button markup, styles, i18n

Replace the old fullscreen localization key and button implementation with an iconized, accessible button and updated styling. Removed fullscreenBtn entries from locale files and updated js/i18n.js to set the fullscreen button's aria-label using tipFullscreen. popup.html now wraps the button in a header and includes an SVG icon; main.css adjusts layout (overflow, sticky header, button positioning, sizing, and hiding in fullscreen) and tweaks editor input spacing for the new layout.

_locales/ja/messages.json

@@ -164,13 +164,10 @@
   "langKorean": {
     "message": "한국어"
   },
-  "fullscreenBtn": {
-    "message": "全画面"
-  },
   "tipFullscreen": {
     "message": "全画面で編集"
   },
   "tipResize": {
     "message": "ドラッグで拡大/縮小（最小：初期サイズ）"
   }
-}
+}