feat(panel): add sponsor slots fed from sponsors.sanaei.dev

Monthly sponsor placements need to change without cutting a panel
release. Panels now read 3X/sponsors.json from the MHSanaei/sponsors
repo (GitHub Pages on sponsors.sanaei.dev) and show active sponsors in
four slots: an overview banner, a rotating sidebar card (max three), the
login page and a new Sponsors page that also lists open placements.

An entry shows only while enable is not false and until is in the
future; links must be https and logos are png/webp/jpg by name only.
The list is cached for an hour and the last good copy survives upstream
failures; logos are proxied through /sponsors/logo/:name with failures
cached, so CSP stays 'self' and admin browsers never reach a third
party. Admins can hide a slot for 24h. Under XUI_DEBUG the panel reads
a sibling ../sponsors/3X checkout so edits can be previewed before push.

frontend/src/lib/sponsors.ts

@@ -0,0 +1,62 @@
+import type { Sponsor } from '@/generated/types';
+
+export type SponsorSlot = 'dashboard' | 'sidebar' | 'page' | 'login';
+
+const DISMISS_KEY = 'xui.sponsor.dismissed';
+export const SPONSOR_DISMISS_MS = 24 * 60 * 60 * 1000;
+
+export function pickLocale(map: Record<string, string> | undefined, lang: string): string {
+  if (!map) return '';
+  const short = lang.split('-')[0].toLowerCase();
+  return map[lang] || map[short] || map.en || '';
+}
+
+export function sponsorsForSlot(sponsors: Sponsor[], slot: SponsorSlot): Sponsor[] {
+  return sponsors.filter((s) => s.slots.includes(slot));
+}
+
+function readDismissed(): Record<string, number> {
+  try {
+    const parsed: unknown = JSON.parse(localStorage.getItem(DISMISS_KEY) || '{}');
+    return parsed && typeof parsed === 'object' ? (parsed as Record<string, number>) : {};
+  } catch {
+    return {};
+  }
+}
+
+export function isSponsorDismissed(id: string, slot: SponsorSlot, now = Date.now()): boolean {
+  const at = readDismissed()[`${id}:${slot}`];
+  return typeof at === 'number' && now - at < SPONSOR_DISMISS_MS;
+}
+
+export function dismissSponsor(id: string, slot: SponsorSlot, now = Date.now()) {
+  const next = Object.fromEntries(
+    Object.entries(readDismissed()).filter(([, at]) => now - at < SPONSOR_DISMISS_MS),
+  );
+  next[`${id}:${slot}`] = now;
+  try {
+    localStorage.setItem(DISMISS_KEY, JSON.stringify(next));
+  } catch {}
+}
+
+// Mirrors the backend: dashboard/login show one sponsor, the sidebar rotates up to three.
+export const SLOT_CAPACITY: Partial<Record<SponsorSlot, number>> = {
+  dashboard: 1,
+  login: 1,
+  sidebar: 3,
+};
+
+export interface PlacementStatus {
+  count: number;
+  capacity?: number;
+  takenUntil?: string;
+}
+
+// When full, a place frees up once enough bookings end to drop below capacity.
+export function placementStatus(sponsors: Sponsor[], slot: SponsorSlot): PlacementStatus {
+  const booked = sponsorsForSlot(sponsors, slot);
+  const capacity = SLOT_CAPACITY[slot];
+  if (!capacity || booked.length < capacity) return { count: booked.length, capacity };
+  const ends = booked.map((s) => s.until).sort((a, b) => Date.parse(a) - Date.parse(b));
+  return { count: booked.length, capacity, takenUntil: ends[booked.length - capacity] };
+}