New:
- Options page: a dedicated options UI for configuring the extension
- Options page can be accessed via the extensions addon page in "about:addons"
- Enable/Disable: a top-level toggle lets you turn the extension off without disabling or uninstalling it in the browser addons menu
- Duplicate behaviour: choose between "Do nothing", "Keep older", or "Keep newer" when a duplicate is opened
- Prefer closing unloaded tabs: optional setting that will close discarded/unloaded tabs first
- Pinned tabs behaviour: choose how pinned tabs are treated (default: never close a pinned tab)
- Keep explicit duplicates: duplicates created from the active tab can be preserved
- Icon click behaviour: pick what happens when you click the toolbar icon (open options, toggle extension, or run cleanup)
- "Close all duplicates now" button: manual cleanup button that closes duplicates immediately (keeps one pinned tab per URL if there are any)
- UI theme for options page: dark mode is the default and a light-mode toggle is available in the options

Fixes:
- Manifest: removed unsupported manifest keys to silence install-time warnings

Known Issues:
- Options UI is ugly (I'm no artist)

Updated documentation to refer to the correct PRIVACY.md (which is https://github.com/sonnenfell/firefox-new-duplicate-tab-closer/blob/main/PRIVACY.md). This doesn't have any effect on the extensions functionality, which is why I didn't change the version number.