feat(panel): let a sponsor slot start at a scheduled time

sponsors.json only had an end date, so a booked placement had to be
added to the file on the day it started. An optional `from` now hides
a sponsor (and its logo) until that instant; entries without it show
immediately as before.

frontend/src/generated/zod.ts

@@ -1091,6 +1091,7 @@ export type Setting = z.infer<typeof SettingSchema>;
 
 export const SponsorSchema = z.object({
   enable: z.boolean().nullable().optional(),
+  from: z.string().nullable().optional(),
   id: z.string(),
   link: z.string(),
   logo: z.string().optional(),