feat(install): write launcher language marker files

Some games include a language.txt marker in the unpacked local tree, similar
in spirit to account_name.txt. Installs and updates now carry the launcher
language alongside the account name so those game-provided marker files are
rewritten before staged files are promoted into local/.

The Tauri command boundary keeps the UI setting vocabulary as de/en, then maps
it to the file vocabulary expected by games: german or english. Unknown values
continue through the existing DEFAULT_LANGUAGE path, so the marker file falls
back to english just like script launch arguments fall back to en.

The transaction layer deliberately reuses the same first-match traversal helper
for both marker files. The searches stay independent, so games may place
account_name.txt and language.txt in different directories if their archive
layout requires that.

Test Plan:
- just fmt
- just test
- just frontend-test
- just clippy
- deno task build
- git diff --check

Refs: none

crates/lanspread-peer-cli/src/main.rs

@@ -241,6 +241,7 @@ async fn handle_command(
                 file_descriptions: files,
                 install_after_download: *install_after_download,
                 account_name: None,
+                language: None,
             })?;
             Ok(json!({"queued": true, "game_id": game_id, "install": install_after_download}))
         }
@@ -250,6 +251,7 @@ async fn handle_command(
             sender.send(PeerCommand::InstallGame {
                 id: game_id.clone(),
                 account_name: None,
+                language: None,
             })?;
             Ok(json!({"queued": true, "game_id": game_id}))
         }