fix(calendar): all-day end.date must be next day, clear opposite field

Google Calendar API rejects all-day events where start.date == end.date
(end is exclusive). POST/PUT were sending the same date for both,
producing Invalid start time when toggling Весь день on edit.

- Added nextDayISO helper (UTC-safe +1d arithmetic)
- all-day: start = { date }, end = { date: nextDay, dateTime: null }
- timed: also explicitly nulls start.date/end.date so patching a
  timed-only event over a previously all-day one doesnt leave stale
  date fields that also trigger Invalid start time

app/api/calendar/route.ts

@@ -4,6 +4,12 @@ import { google } from 'googleapis'
 import * as fs from 'fs'
 import * as path from 'path'
 
+function nextDayISO(date: string): string {
+  const d = new Date(`${date}T00:00:00Z`)
+  d.setUTCDate(d.getUTCDate() + 1)
+  return d.toISOString().slice(0, 10)
+}
+
 function getAuth(readonly = true) {
   const scopes = readonly
     ? ['https://www.googleapis.com/auth/calendar.readonly']
@@ -134,11 +140,11 @@ export async function POST(req: Request) {
 
   let start: any, end: any
   if (allDay) {
-    start = { date }
-    end = { date }
+    start = { date, dateTime: null }
+    end = { date: nextDayISO(date), dateTime: null }
   } else {
-    start = { dateTime: `${date}T${startTime}:00`, timeZone: 'Europe/Moscow' }
-    end = { dateTime: `${date}T${endTime}:00`, timeZone: 'Europe/Moscow' }
+    start = { dateTime: `${date}T${startTime}:00`, timeZone: 'Europe/Moscow', date: null }
+    end = { dateTime: `${date}T${endTime}:00`, timeZone: 'Europe/Moscow', date: null }
   }
 
   try {
@@ -181,11 +187,11 @@ export async function PUT(req: Request) {
 
   let start: any, end: any
   if (allDay) {
-    start = { date }
-    end = { date }
+    start = { date, dateTime: null }
+    end = { date: nextDayISO(date), dateTime: null }
   } else {
-    start = { dateTime: `${date}T${startTime}:00`, timeZone: 'Europe/Moscow' }
-    end = { dateTime: `${date}T${endTime}:00`, timeZone: 'Europe/Moscow' }
+    start = { dateTime: `${date}T${startTime}:00`, timeZone: 'Europe/Moscow', date: null }
+    end = { dateTime: `${date}T${endTime}:00`, timeZone: 'Europe/Moscow', date: null }
   }
 
   try {