Multi-chat management: pane tabs, session multi-select, and richer session navigation (#697)

* fix(web-ui): give the highlighted tab strip a real session-drop target

* feat(web-ui): show conversation titles in browser tabs

* feat(web-ui): path-based session URLs /s/<id>

* feat(web-ui): archive button in pane tabs — archives, updates Recents, closes the tab

* feat(web-ui): sidebar session multi-select

* feat(web-ui): jump to the Nth recent session with Cmd/Ctrl+1-9

* fix(web-ui): restored panes hydrate the viewer's own sessions instead of flashing a read-only stub

* Remove Cmd/Ctrl 1-9 session jump hotkey

* Show drop targets from drag start for native tab drags

* De-export isMac after session-jump removal

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Co-authored-by: Sprite <noreply@sprites.dev>