Add FireflyGo sync mode, extra_data controls and Enka import

- js/modes.js: per-backend capability matrix (GaCy GO / FireflyGo / RobinSR)
- js/firefly.js: POST /sync payload builder + extra_data normaliser
- js/enka-import.js: enka.network -> FreeSR converter with merge/replace
- app.js: mode-aware UI, firefly transport, Enka import, extra_data form
- index.html: enable FireflyGo, add Enka import + FireflyGo controls cards
- i18n: Thai/English strings for every new label, message and error
- vercel.json: /api/enka/:uid rewrite; scripts/dev-server.cjs for local dev

js/avatars-data.js

@@ -1,4 +1,4 @@
-import { getCurrentLanguage, translateElement, translatePath } from "./i18n.js?v=2.22";
+import { getCurrentLanguage, translateElement, translatePath } from "./i18n.js?v=2.23";
 
 export const AVATAR_METADATA = {
   "1001": {