[AlxcBTW]

Ready for batch.

Every page the launcher opens on the cloud (the sign-in page, the account web, the appeal page) now carries lang=<code> with the language the launcher is set to (en, tr, de, es, fr, pt). The web (anyplaynet/web#48) picks its language from that parameter first, then a cookie, then Accept-Language.

• One place: OnlineService::open_cloud_page, after the existing check that the address is under the player's cloud. Nothing else about which addresses open changes.
• with_language keeps an existing lang= and the fragment, and handles an empty query. Unit test a_cloud_page_opens_in_the_launchers_language.
• Checked through tools/lb: cargo fmt --all -- --check, cargo clippy -p anyps5-launcher --all-targets -- -D warnings, and the new test pass.
• An older web that does not know lang ignores the parameter.

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