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/src/lib.rs

@@ -230,6 +230,7 @@ pub enum PeerCommand {
         id: String,
         file_descriptions: Vec<GameFileDescription>,
         account_name: Option<String>,
+        language: Option<String>,
     },
     /// Download game files with an explicit install policy.
     DownloadGameFilesWithOptions {
@@ -237,11 +238,13 @@ pub enum PeerCommand {
         file_descriptions: Vec<GameFileDescription>,
         install_after_download: bool,
         account_name: Option<String>,
+        language: Option<String>,
     },
     /// Install already-downloaded archives into `local/`.
     InstallGame {
         id: String,
         account_name: Option<String>,
+        language: Option<String>,
     },
     /// Remove only the `local/` install for a game.
     UninstallGame { id: String },
@@ -411,6 +414,7 @@ async fn handle_peer_commands(
                 id,
                 file_descriptions,
                 account_name,
+                language,
             } => {
                 handle_download_game_files_command(
                     ctx,
@@ -419,6 +423,7 @@ async fn handle_peer_commands(
                     file_descriptions,
                     true,
                     account_name,
+                    language,
                 )
                 .await;
             }
@@ -427,6 +432,7 @@ async fn handle_peer_commands(
                 file_descriptions,
                 install_after_download,
                 account_name,
+                language,
             } => {
                 handle_download_game_files_command(
                     ctx,
@@ -435,11 +441,16 @@ async fn handle_peer_commands(
                     file_descriptions,
                     install_after_download,
                     account_name,
+                    language,
                 )
                 .await;
             }
-            PeerCommand::InstallGame { id, account_name } => {
-                handle_install_game_command(ctx, tx_notify_ui, id, account_name).await;
+            PeerCommand::InstallGame {
+                id,
+                account_name,
+                language,
+            } => {
+                handle_install_game_command(ctx, tx_notify_ui, id, account_name, language).await;
             }
             PeerCommand::UninstallGame { id } => {
                 handle_uninstall_game_command(ctx, tx_notify_ui, id).await;