feat(ui): customizable sidebar — collapsible, reorderable, toggleable widgets

Each filter group is now a card with a collapse chevron. An Edit mode (pencil)
reveals drag handles (@dnd-kit) to reorder and eye toggles to show/hide widgets,
plus Reset to defaults. Layout (order/collapsed/hidden) persists to localStorage
and the server preferences blob, adopted on login.

frontend/src/lib/sidebarLayout.ts

@@ -0,0 +1,64 @@
+// Per-user customization of the filter sidebar: order of the widget cards, which are
+// collapsed, and which are hidden. Persisted to localStorage and the server-side
+// `preferences` blob so it follows the account.
+
+export type WidgetId = "show" | "sort" | "date" | "content" | "language" | "topic";
+
+export const ALL_WIDGETS: WidgetId[] = [
+  "show",
+  "sort",
+  "date",
+  "content",
+  "language",
+  "topic",
+];
+
+export const WIDGET_TITLES: Record<WidgetId, string> = {
+  show: "Show",
+  sort: "Sort",
+  date: "Upload date",
+  content: "Content type",
+  language: "Language",
+  topic: "Topic",
+};
+
+export interface SidebarLayout {
+  order: WidgetId[];
+  collapsed: Partial<Record<WidgetId, boolean>>;
+  hidden: Partial<Record<WidgetId, boolean>>;
+}
+
+export const DEFAULT_LAYOUT: SidebarLayout = {
+  order: [...ALL_WIDGETS],
+  collapsed: {},
+  hidden: {},
+};
+
+const KEY = "subfeed.sidebarLayout";
+
+// Tolerate stale/partial data: keep only known widgets and append any that are missing
+// (e.g. a widget added in a later version) so nothing silently disappears.
+export function normalizeLayout(raw: unknown): SidebarLayout {
+  const r = (raw ?? {}) as Partial<SidebarLayout>;
+  const order: WidgetId[] = Array.isArray(r.order)
+    ? (r.order.filter((x) => ALL_WIDGETS.includes(x as WidgetId)) as WidgetId[])
+    : [];
+  for (const w of ALL_WIDGETS) if (!order.includes(w)) order.push(w);
+  return {
+    order,
+    collapsed: { ...(r.collapsed ?? {}) },
+    hidden: { ...(r.hidden ?? {}) },
+  };
+}
+
+export function loadLayout(): SidebarLayout {
+  try {
+    return normalizeLayout(JSON.parse(localStorage.getItem(KEY) || "{}"));
+  } catch {
+    return DEFAULT_LAYOUT;
+  }
+}
+
+export function saveLayoutLocal(l: SidebarLayout): void {
+  localStorage.setItem(KEY, JSON.stringify(l));
+}