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

package.json

@@ -8,7 +8,8 @@
     "sync-build-guides": "node scripts/sync-build-guides.mjs",
     "dev": "node scripts/sync-branch.js && npx serve . -l 5173",
     "start": "node scripts/sync-branch.js && npx serve . -l 5173",
-    "serve": "npx serve . -l 5173"
+    "serve": "npx serve . -l 5173",
+    "dev:proxy": "node scripts/sync-branch.js && node scripts/dev-server.cjs"
   },
   "keywords": [
     "hkrpg",