Improve workflow for forward/backward navigation

I removed the forward/backward navigation arrows in 3.4.4135 (and put them back in 3.4.4137) to identify how widely they were used. It turns out they were more widely used than I thought! There are a few issues with them, and I was hoping to address a few and just let the rest be covered by other fallbacks:

• They take up space on the tab bar, which is going to be at a premium for both Project and Enterprise workflows that encourage multiple tabs with more information per tab.
• They are confusing, since they appear to be for scrolling through tabs, rather than navigating in the current view.
• They interact oddly with sync groups on panes.

After some discussion, the proposal now is to make navigation history saved per sync group. We can then move the forward/backward arrows to each pane and make them work per sync group. This should address all of the concerns above.

If you are also someone that cares about forward/backward navigation, and the proposal above will break your workflow or isn't sufficient to address a concern I wasn't tracking, please comment on this ticket and let me know.

There is no current timeline for these changes, it's just something I'm tracking as part of general UX improvements going forward.

No files, tests, or entry points are named. Start by locating the navigation-history and sync-group behavior for panes, then determine how per-sync-group history and pane-level controls should fit together; done means the proposed workflow addresses tab-bar space, control clarity, and sync-group interaction.