fix: drop phantom ended-tab stubs, trust server as source of truth (#93)

Previously the client cached open session tabs in localStorage and resurrected
any that the server no longer knew about as grayed-out "ended" stubs. On
multi-device use (close tab on mobile, open desktop) this left stale phantom
tabs the user had to manually dismiss.

Remove _restoreEndedTabs / _saveTabMetadata, the session._ended branch in
selectSession, the data-ended render attribute, and the matching CSS rule.
Clear the legacy localStorage key on init to purge stale entries.

Co-authored-by: Teigen <teigen@TeigendeMac-mini.local>