Git's `-d` flag deletes a branch only if it's fully merged into its
upstream or HEAD - this is what we were using before, which caused the
"not fully merged" error. The `-D` flag force deletes a branch even with
unmerged changes (equivalent to `--delete --force`).

### Before

Deleting an unmerged branch failed with a "not fully merged" error
toast.

### After

- Deleting an unmerged branch prompts for confirmation to force delete
- Delete button tooltip shows "Hold alt to force delete" hint
- Holding **alt** turns the delete icon red and tooltip changes to
"Force Delete Branch"
- Force delete keybinding: `cmd-alt-shift-backspace`

Release Notes:

- Added confirmation prompt when deleting unmerged git branches, with
option to force delete.
- Added alt+click on delete button to force delete a branch immediately.