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/examples.ts

@@ -1107,6 +1107,7 @@ export const EXAMPLES: Record<string, unknown> = {
   },
   "Sponsor": {
     "enable": true,
+    "from": "2026-10-01T00:00:00Z",
     "id": "acme-2026-10",
     "link": "https://acme.example/?utm_source=3x-ui",
     "logo": "/sponsors/logo/acme.png",
@@ -1123,6 +1124,7 @@ export const EXAMPLES: Record<string, unknown> = {
     "sponsors": [
       {
         "enable": true,
+        "from": "2026-10-01T00:00:00Z",
         "id": "acme-2026-10",
         "link": "https://acme.example/?utm_source=3x-ui",
         "logo": "/sponsors/logo/acme.png",