tabs: keep all tab content mounted (inactive hidden), so switching tabs never reloads a terminal

Switching tabs used to unmount the active view; a terminal's unmount sent
the tab-close frame, killing its pty, and switching back respawned a fresh
shell. Every tab now stays mounted and inactive ones are display:none: a
terminal keeps its WebSocket + pty + scrollback, an editor keeps its
CodeMirror view and unsaved draft, explorer/git keep their data. The close
frame now fires only when a tab is truly closed (close button, middle
click, pane removal). Re-shown terminals re-fit via the existing
ResizeObserver; editors re-measure via CodeMirror's own observer.

src/client/Sidebar.tsx

@@ -205,7 +205,6 @@ export function Sidebar(props: { ctx: Context }) {
 
   const renderTab = (tab: SidebarTab) => (
     <TabContent
-      key={tab.id}
       tab={tab}
       sessionId={sessionId}
       cwd={cwd}