feat(mouse-gesture): add option to Show Previously Selected Tab

[Leblayd]

Implementing the missing option mentioned (by me) in #2158

[Leblayd]

I quickly tested it locally, and it seems to work as expected.

Technically it'd run into an error if the number of tabs were 0, but that seems impossible to me. Otherwise, it should work just like Gesturefy.

The localization stuff confuses me a little bit, though.
Should I add translations for languages I speak?
And should I add the key translated to English for all the other languages as well, or leave them out so it falls back to the translation key?

[surapunoyousei]

Hi. @Leblayd Thank you for your contribution!

[Copilot]

Pull request overview

This PR implements a new mouse gesture and keyboard shortcut action to switch to the previously selected tab based on access time. The feature adds the gecko-show-previously-selected-tab command that finds the most recently accessed tab (excluding the current one) and switches to it.

Key Changes:

• Added gecko-show-previously-selected-tab command that iterates through tabs to find the one with the highest _lastAccessed timestamp (excluding tabs with Infinity)
• Registered the new action in both the custom shortcut key system and mouse gesture system
• Made the action available in the gesture settings UI

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
libs/shared/custom-shortcut-key/commands.ts	Added the gecko-show-previously-selected-tab command with logic to find and select the previously accessed tab
browser-features/chrome/common/mouse-gesture/utils/actions.ts	Registered the same action for mouse gestures with identical logic (but includes debug logging and a return statement)
browser-features/pages-settings/src/app/gesture/useAvailableActions.ts	Added the new action to the list of available gesture actions

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