refactor: rename frontend/ to web-player/ and format with prettier

Updates the compose build context, the Dockerfile header, the package
name and the docs. The compose service and container keep the name
frontend/musify-frontend, since nginx routes to http://frontend:3000.

The source files also carry the first prettier pass, so the rename and
the reformat land together for them.

web-player/.env.example

@@ -0,0 +1,23 @@
+# Zitadel OIDC (server-side auth)
+ZITADEL_ISSUER=http://host.docker.internal:8080
+ZITADEL_CLIENT_ID=your-web-app-client-id
+ZITADEL_CLIENT_SECRET=your-web-app-client-secret
+
+# Where Zitadel redirects back after login (must be registered in the app)
+AUTH_REDIRECT_URI=http://localhost:5173/auth/callback
+AUTH_POST_LOGOUT_URI=http://localhost:5173/
+
+# Secret used to derive the session cookie encryption key (any long random string)
+SESSION_SECRET=change-me-to-a-long-random-string
+
+# Optional session tuning (defaults shown)
+# OIDC_SCOPE=openid profile email offline_access
+# SESSION_TTL_SECONDS=604800
+# SESSION_REFRESH_THRESHOLD_SECONDS=30
+# AUTH_FLOW_TTL_SECONDS=600
+
+# Dev already allows a plain-HTTP issuer. Set this only to allow it in a production build (never with a public issuer).
+# AUTH_ALLOW_INSECURE_HTTP=true
+
+# Backend API base URL (used from later parts)
+API_BASE_URL=http://localhost:5111