refactor: cf-only deploy via dashboard, drop gh actions

- Default `npm run build` now produces a root-relative build (CF Pages
  custom domain at loto.miti99.com). The /loto basePath is opt-in via
  `npm run build:gh` for the rare manual GH Pages export.
- Removed both GitHub Actions deploy workflows (.github/workflows/) and
  the dangling `build:cf` script (it was identical to `build` after the
  default flip).
- next.config.mjs: simplified basePath logic — only `BUILD_PROFILE=gh`
  toggles a non-empty path; everything else (CF, local dev) is root.

web/package.json

@@ -7,7 +7,6 @@
     "dev:codeserver": "NEXT_DEV_PROFILE=codeserver next dev -H 0.0.0.0",
     "build": "next build",
     "build:gh": "BUILD_PROFILE=gh next build",
-    "build:cf": "BUILD_PROFILE=cf next build",
     "start": "next start",
     "lint": "eslint"
   },