fix(i18n): add missing dialogs.share_folder translation key

The share dialog for folders was failing because dialogs.share_folder
key was missing from all locale files. The contextMenus.js code
references this key when opening the share dialog for folders:

  window.i18n.t('dialogs.share_folder')

Added the missing key to all 14 locale files with appropriate
translations for each language.

Fixes #189

static/locales/zh.json

@@ -175,6 +175,7 @@
     "and_contents": "及其所有内容",
     "no_undo": "此操作无法撤销",
     "share_file": "共享文件",
+    "share_folder": "共享文件夹",
     "existing_shares": "现有共享",
     "share_options": "共享选项",
     "password": "密码",