build: separate .env.build for packaging, keep .env dev-only

The previous setup packaged the development `.env` into the installer
resources, mixing local server settings (PORT/HOST/SESSION_SECRET) with
the build-time site domain. Introduce a dedicated `.env.build`:

- electron-builder configs now copy `.env.build` (gitignored) into
  `resources/`, no longer touching the dev `.env`.
- `loadEnv()` prefers `resources/.env.build` first, falling back to
  `resources/.env` (for operators who hand-edit the packaged file),
  then `<root>/.env.build`, then `<root>/.env`.
- `.env.build.example` documents the build-only keys (SITE_BASE_URL,
  MANIFEST_URL, MUSIC_CONCURRENCY); server-side keys stay in `.env`.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

electron-builder-rp.yml

@@ -13,12 +13,12 @@ files:
   - installer-rp/**
   - build/icon.*
   - package.json
-# 메인 설치기와 동일하게 .env, locales 를 함께 배포.
+# 메인 설치기와 동일하게 빌드 전용 `.env.build` 와 locales 를 함께 배포.
 extraResources:
   - from: .
     to: .
     filter:
-      - .env
+      - .env.build
   - from: locales
     to: locales
     filter: