feat(admin): event form parity with the webapp (1.6.1-aio.13)

The LNbits admin form lagged the webapp's CreateEventDialog:

- Payment methods never rendered. c2d9a96 wired the template to
  `paymentMethodOptions` / `acceptsFiat` but never defined them, so the
  q-option-group got `options=undefined` and the fiat-currency select
  was gated on `undefined`. Rails are now two q-checkboxes; Card is
  disabled with an explanatory tooltip when `g.user.fiat_providers` is
  empty (same rule as the webapp) and names the providers otherwise.
- Location (NIP-52 `location` tag) and Categories (NIP-52 `t` tags,
  same 25-item list as the webapp's category.ts) were missing from the
  form even though the model, CRUD and publisher already carry them.
- Datetimes are stamped with the browser's UTC offset on submit, as the
  webapp does; `_to_unix` treats naive values as UTC, so 18:00 CEST
  entered here went out on Nostr as 18:00 UTC. Table columns render
  "YYYY-MM-DD HH:MM" instead of the raw ISO string.
- Validation: title + start date required, end >= start on the folded
  date+time, fiat currency required when a sat-priced event accepts
  card. Create is enabled once wallet + title + start are set; info,
  closing date, tickets and price were all effectively required before
  because the disable check compared undefined fields to null.
- Labels follow the payment-rails vocabulary: "Unit" -> "Price
  currency", "Fiat checkout currency" -> "Fiat currency"; ticket
  closing date and end date explain their defaults.
- A fiat-priced event mirrors `fiat_currency = currency` on save so the
  payload and the `tickets_fiat_currency` tag stay coherent.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018b1bDExMX7W3a47wcgFUjb

config.json

@@ -1,6 +1,6 @@
 {
   "id": "events",
-  "version": "1.6.1-aio.10",
+  "version": "1.6.1-aio.13",
   "name": "Events",
   "repo": "https://git.atitlan.io/aiolabs/events",
   "short_description": "Sell and register event tickets",