Add MRU tab switching (last-used tab)

Windows Alt+Tab and FF Ctrl+Tab support switching between two tabs, even if they are spaced apart. Some Chrome solutions are linked [here](https://superuser.com/a/433532/269989). However, TabFern already has all the information it needs to provide this function at a basic level, so why load a separate extension?

- [ ] Add command for MRU switching
- [ ] Add an array to track MRU per window
- [ ] When the key shortcut is pressed, traverse the MRU array for the current window.

Open question: what to do if the TabFern popup has the focus? Switch to the active tab in the last-focused non-popup window?

No files or tests are named. Start by locating the existing command and shortcut handling, then find how tabs and windows are tracked; implement the MRU behavior for the current window and define what should happen when the TabFern popup has focus. Done means a shortcut can switch to the last-used tab, including tabs that are not adjacent.

Written by the indexing model from the issue text.