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.module.css

@@ -269,6 +269,19 @@
   overflow: hidden;
 }
 
+/* One tab's mounted content; inactive tabs stay mounted but hidden so
+   switching never tears down terminals/editors (state survives). */
+.paneTab {
+  flex: 1;
+  min-height: 0;
+  display: flex;
+  flex-direction: column;
+}
+
+.paneTabHidden {
+  display: none;
+}
+
 .paneEmptyCards {
   flex: 1;
   min-height: 0;