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

vercel.json

@@ -1,4 +1,10 @@
 {
   "buildCommand": "node scripts/build-vercel.cjs",
-  "outputDirectory": "dist"
+  "outputDirectory": "dist",
+  "rewrites": [
+    {
+      "source": "/api/enka/:uid",
+      "destination": "https://enka.network/api/hsr/uid/:uid"
+    }
+  ]
 }