Add launcher profile settings for username and language, then thread those
values into the Windows script launch path. The game setup, game start, and
server start scripts now share the same argument shape:
- game path: local
- game id
- language: en or de
- player name
Expose a local can_host_server flag in the games payload by checking for
server_start.cmd in an installed game's root directory. The detail modal uses
that flag to show Start Server only for installed games with the script, and
the new start_server command invokes server_start.cmd with the same sanitized
settings used by game_setup.cmd and game_start.cmd.
Test Plan:
- just fmt
- just test
- just frontend-test
- just build
- just clippy
- git diff --check
Refs: design/README.md
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