fix(panel): add the mothership-panel store missing from the resize commit

The persisted split-width store (stores/mothership-panel) never made it
into "the split remembers" — the branch referenced a module that only
existed in the working tree.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: andresdjasso

apps/sim/stores/mothership-panel/store.ts

@@ -0,0 +1,32 @@
+import { create } from 'zustand'
+import { devtools, persist } from 'zustand/middleware'
+
+/**
+ * Persisted split geometry for the Mothership surfaces, so a user's resize
+ * survives view switches, route changes, and reloads. Widths are px;
+ * `null` means the user never resized (CSS defaults apply). Consumers clamp
+ * to the current viewport on apply — never at save time.
+ */
+interface MothershipPanelState {
+  /** Resource panel width on the chat surface. */
+  panelWidth: number | null
+  /** Docked chat pane width on the workflow route. */
+  chatPaneWidth: number | null
+  setPanelWidth: (width: number) => void
+  setChatPaneWidth: (width: number) => void
+}
+
+export const useMothershipPanelStore = create<MothershipPanelState>()(
+  devtools(
+    persist(
+      (set) => ({
+        panelWidth: null,
+        chatPaneWidth: null,
+        setPanelWidth: (width) => set({ panelWidth: width }),
+        setChatPaneWidth: (width) => set({ chatPaneWidth: width }),
+      }),
+      { name: 'mothership-panel' }
+    ),
+    { name: 'mothership-panel-store' }
+  )
+)