fix(activities): stamp local tz offset on event datetimes before submit

The form sent naive "YYYY-MM-DDTHH:MM" to the LNbits events backend,
where _to_unix (nostr_publisher.py) assumes UTC when tzinfo is None.
So 08:00 entered in CEST got stored as 08:00 UTC, and the NIP-52 start
tag landed on the relays at the wrong instant — the detail page then
re-localized it to 10:00 (offset doubly applied).

Stamp the wall-clock value with the user's UTC offset before sending so
the backend builds the correct unix and the detail page renders the
intended wall-clock. Seconds (`:00`) included for pre-3.11 Python
fromisoformat compatibility. Round-trips through edit mode unchanged:
splitDateTime trims to "HH:MM" so the suffix drops cleanly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/modules/activities/components/CreateEventDialog.vue

@@ -87,6 +87,28 @@ function foldDateTime(date: string, time: string): string {
   return time ? `${date}T${time}` : date
 }
 
+// Stamp the form's wall-clock datetime with the user's local UTC offset
+// before sending it to the LNbits events backend. Without this, the
+// backend's `_to_unix` (nostr_publisher.py) treats a naive ISO string
+// as UTC, so e.g. "08:00" entered in CEST gets stored as 08:00 UTC and
+// the NIP-52 `start` tag is off by the user's offset on the relay
+// — the detail page then renders it +offset (08:00 → 10:00 in CEST).
+// Preserving the user's intended wall-clock means stamping it here.
+// Date-only values (no "T") pass through unchanged.
+function withLocalTzOffset(value: string): string {
+  if (!value || !value.includes('T')) return value
+  // The form's "YYYY-MM-DDTHH:MM" is parsed by JS Date as local time;
+  // getTimezoneOffset() returns minutes west of UTC, so negate it.
+  const offMin = -new Date(value).getTimezoneOffset()
+  const sign = offMin >= 0 ? '+' : '-'
+  const abs = Math.abs(offMin)
+  const hh = String(Math.floor(abs / 60)).padStart(2, '0')
+  const mm = String(abs % 60).padStart(2, '0')
+  // Include `:00` seconds for compatibility with older Python
+  // `datetime.fromisoformat` (pre-3.11 won't accept "HH:MM+HH:MM").
+  return `${value}:00${sign}${hh}:${mm}`
+}
+
 const formSchema = toTypedSchema(
   z
     .object({
@@ -138,8 +160,11 @@ interface BannerImage extends UploadedImage {
 }
 const bannerImages = ref<BannerImage[]>([])
 
-// Inverse of foldDateTime: split a stored "YYYY-MM-DD[THH:MM]" back
-// into separate date + time pieces for the form inputs.
+// Inverse of foldDateTime: split a stored "YYYY-MM-DD[THH:MM[:SS][±HH:MM]]"
+// back into separate date + time pieces for the form inputs. The
+// time slice trims to "HH:MM" so any seconds + offset suffix added by
+// withLocalTzOffset on submit drops cleanly — the user sees the same
+// wall-clock they originally entered when re-editing.
 function splitDateTime(value: string | null | undefined): { date: string; time: string } {
   if (!value) return { date: '', time: '' }
   const [date, time = ''] = value.split('T')
@@ -267,9 +292,8 @@ const onSubmit = form.handleSubmit(async (formValues) => {
   try {
     const eventData: CreateEventRequest = {
       name: formValues.name,
-      event_start_date: foldDateTime(
-        formValues.event_start_date,
-        formValues.event_start_time
+      event_start_date: withLocalTzOffset(
+        foldDateTime(formValues.event_start_date, formValues.event_start_time)
       ),
     }
     if (!isEditMode.value) {
@@ -281,9 +305,8 @@ const onSubmit = form.handleSubmit(async (formValues) => {
     // Optional fields — only include if provided
     if (formValues.info) eventData.info = formValues.info
     if (formValues.event_end_date) {
-      eventData.event_end_date = foldDateTime(
-        formValues.event_end_date,
-        formValues.event_end_time
+      eventData.event_end_date = withLocalTzOffset(
+        foldDateTime(formValues.event_end_date, formValues.event_end_time)
       )
     }
     if (formValues.location) eventData.location = formValues.location