fix(web): cache empty shells array in TerminalTabs selector

useWorkspaceUiStore((s) => s.shellsByWorkspace[workspaceId] ?? [])
returned a fresh [] on every render when the workspace had no shells
yet, so useSyncExternalStore saw a new snapshot each render and
re-rendered, which ran the selector again, ad infinitum.

  "The result of getSnapshot should be cached to avoid an infinite
   loop"
  TerminalTabs @ WorkspaceShell.tsx:75

Fix: hoist a module-level EMPTY_SHELLS constant. The selector now
returns a stable reference for the empty case. The non-empty case
already returns the array stored in the Zustand store, whose
reference is set once per setShellsForWorkspace call.

web/src/components/layout/WorkspaceShell.tsx

@@ -12,6 +12,12 @@ import { FileExplorerPanel } from "@/components/workspace/FileExplorerPanel";
 import { StatusBar } from "@/components/workspace/StatusBar";
 import { WorkspaceSelector } from "@/components/workspace/WorkspaceSelector";
 import { useWorkspaceUiStore } from "@/lib/store/workspace-ui-store";
+
+// Module-level constant so the selector returns a stable reference when
+// the workspace has no shells yet. Returning a freshly-allocated `[]` from
+// the selector (via `?? []`) would make useSyncExternalStore see a new
+// snapshot on every render and loop.
+const EMPTY_SHELLS: string[] = [];
 import {
   useProcessRestart,
   useProcessStart,
@@ -72,7 +78,7 @@ function BottomTabs({ workspaceId }: { workspaceId: string }) {
 }
 
 function TerminalTabs({ workspaceId }: { workspaceId: string }) {
-  const shells = useWorkspaceUiStore((s) => s.shellsByWorkspace[workspaceId] ?? []);
+  const shells = useWorkspaceUiStore((s) => s.shellsByWorkspace[workspaceId] ?? EMPTY_SHELLS);
   const activeShellId = useWorkspaceUiStore((s) => s.activeShellIdByWorkspace[workspaceId]);
   const setShellsForWorkspace = useWorkspaceUiStore((s) => s.setShellsForWorkspace);
   const addShellToWorkspace = useWorkspaceUiStore((s) => s.addShellToWorkspace);