Use title case for context menu entries on macOS and Linux

Context menu entries should be title-cased, i.e.,

Lock file → Lock File
Share options → Share Options
Always keep downloaded → Always Keep Downloaded
Open in browser → Open in Browser
Copy internal link → Copy Internal Link

Note that this only applies to [macOS](https://developer.apple.com/design/human-interface-guidelines/the-menu-bar#Best-practices) and Linux ([KDE](https://develop.kde.org/hig/text_and_labels/#capitalization-and-punctuation) and [GNOME](https://developer.gnome.org/hig/guidelines/writing-style.html#header-capitalization)), as Windows generally uses sentence case.