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-tauri-deno-ts/src/hooks/useGameActions.ts

@@ -53,6 +53,7 @@ export const useGameActions = (
         try {
             const success = await invoke<boolean>('install_game', {
                 id,
+                language: settings.language,
                 username: settings.username,
             });
             if (!success) return;
@@ -64,19 +65,20 @@ export const useGameActions = (
         } catch (err) {
             console.error('install_game failed:', err);
         }
-    }, [games, settings.username]);
+    }, [games, settings.language, settings.username]);
 
     const update = useCallback(async (id: string) => {
         try {
             const success = await invoke<boolean>('update_game', {
                 id,
+                language: settings.language,
                 username: settings.username,
             });
             if (success) games.markChecking(id);
         } catch (err) {
             console.error('update_game failed:', err);
         }
-    }, [games, settings.username]);
+    }, [games, settings.language, settings.username]);
 
     const uninstall = useCallback(async (id: string) => {
         try {