fix(tabs): reopen a closed tab at its original position

Ctrl/Cmd+Shift+T brought a closed tab back at the end of the strip
instead of where it had been. The closed-tab entry now records the slot
the tab was closed from, and reopening splices it back in there, clamped
to the current strip length, the way a browser restores a tab.

Close-others and close-all record each tab against the strip as it would
be once the earlier members are gone, so repeated reopens walk the strip
back into its original order. Conversation, draft and file tabs all go
through it. Activation, the dedupe of an already-open conversation, the
preview slot rule and the stack cap are unchanged. The reordered strip
persists through the existing opened_tabs save.

Follow-up to #502.