Shell polish: storage line, status pill, footer strip, sidebar

Backend:
- apps.core with GET /api/status/ (env, git hash, OS, watched folder,
  worker counts) and a TimingMiddleware adding X-Server-Time-Ms
- status endpoint reports its own server-side assembly time

Frontend:
- top bar shows the storage line (watched folder) and a status pill
  with ENV, git hash, host OS, server time (e621 time once it exists)
- fixed 32px footer strip: storage path, active workers, build version
- collapsible filter sidebar: closed by default, Ctrl/Cmd+B toggle,
  overlay drawer with backdrop blur under 1024px
- rating filters, per-page and sidebar state persist in localStorage

frontend/src/store/ui.ts

@@ -0,0 +1,29 @@
+import { create } from "zustand";
+import { persist } from "zustand/middleware";
+
+interface UiState {
+  sidebarOpen: boolean;
+  setSidebarOpen: (open: boolean) => void;
+  toggleSidebar: () => void;
+  ratings: string[];
+  setRatings: (ratings: string[]) => void;
+  perPage: number;
+  setPerPage: (perPage: number) => void;
+}
+
+export const useUi = create<UiState>()(
+  persist(
+    (set) => ({
+      sidebarOpen: false,
+      setSidebarOpen: (sidebarOpen) => set({ sidebarOpen }),
+      toggleSidebar: () => set((state) => ({ sidebarOpen: !state.sidebarOpen })),
+
+      ratings: [],
+      setRatings: (ratings) => set({ ratings }),
+
+      perPage: 48,
+      setPerPage: (perPage) => set({ perPage }),
+    }),
+    { name: "j621.ui" },
+  ),
+);